Transcript power_point1..metal_detector..final1_

An_Najah National University
Electrical Engineering Department
Metal Detectors
Prepared by :
Anwar Abu Khazneh
Ola Abu Hijleh
Supervisor : Mazen Rasekh
2011
METAL
DETECTORS
contents
 Principle
of work
 Types of metal detector
 Advantages of pulse induction method
 Components of metal detector
 Things must be observed in designing metal
detector
 Application for metal detector
•
Principle of operation of metal detector:
 The
operation of a metal detector is based on the principle
of electromagnetic induction.
 Metal
detectors contain one or more inductor coils. When
metal is placed in a close proximity to a varying magnetic
field (generated by the coil or coils), currents are induced
in the metallic part .
 These
current are called eddy Currents. The eddy
Currents, in turn, induce their own magnetic field ( called
eddy fields) .
 These
fields act in such a direction as to oppose that
generated by the coils. The resultant field
( H applied – H eddy ) and using a specially designed
electronic circuit can indicate the type of material being
magnetized.
•
Types of Metal detectors:1- Pulse induction detector (PI).
2-Beat-frequency oscillator (BFO) .
3-Very Low Frequency (VLF) .
Very Low Frequency (VLF) .
Uses two coils:
 -Transmitter coil (search head, antenna)
 -Receiver coil
 There is a delay in the received signal
 compared to the transmitted signal
 called phase shift

In our project we use the PI method

Simple when compared to VLF detectors.

Pulse Induction (PI) metal detectors are sending repeated
pulses of electrical current to the search coil, producing a
magnetic field. The coil transmits a pulse toward the
ground, generating an answering pulse from the target
object. A sampling circuit measures the pulse and sends it
to an integrator, which generates an audio tone.

Pulse Induction detectors are able to detect objects buried
deep underground, but they are sensitive to iron
The circuit:
A simplified diagram of the circuit is shown in Fig. 1. It can be
divided into five sections:
1-power supply.
2- clocking.
3-transmit plus receiver front-end.
4-receiver back-end.
5-audio.
Each section
can be built and tested sequentially.
Block diagram
Now I will explain the first part which is
power supply
Power for the circuit is provided by single 12 volt.
 The power supply consists of a voltage doubler.
 And 3 regulators.
Power diagram

The second part which is
clocking

The circuit
clocking

The master clock generator is a 555 timer (IC5).

The 555 oscillator has two controls.R2 varies the
frequency of the main pulse, and R3 varies the pulse
width .

The frequency of the main pulse determine how many
times per second the coil is pulsed.

Generally, ahigher pulse frequency allows for a faster coil
motion and perhaps a little better noise performance,while
consuming more power.

Varying the main pulse width determines how long the coil
is turned on,which can affect depth of detection
Third part of PI circuit
transmit plus receiver front-end.

The main device in a PI detector is the coil switch.

The coil switch serves two functions:
To shirt the coil across the battery voltage,which creates
alarge coil current;
And to turn the coil current off . During the “on” time ,current
flowing through the coil creates a magnetic field around the
coil
R11 is the coil damping resistor and prevent ringing
when Q3 is turns off
Ringing will occur because of the L-C cct created by
the inductance , and capacitance of the coil .

The coil signal is applied to preamp.IC6,which is connected
as an inverting opamp.R13/12 Sets the gain to 1000.

Diodes D3 and D4 provide clamping to protect the opamp .
* The advantages of pulse induction (PI) detectors:Emit an electromagnetic signal of higher intensity and
thus , these signals penetrate far deeper into the ground
than continuously emitted signals.
 Cover large areas in less time.
 Search coil or loop of a pulse induction metal detector is
simpler than VLF instrument.
A single coil of wire is commonly used for both the transmit
and receive functions, while in VLF two coils are needed
transmit coil and receive coil.

the coil:
 we use a mono coil
 Important coil parameter are coil diameter, number of
windings, wire gauge.
 The standard coil for this design has diameter of 10 inches
and 26 turns.
 The diameter affect the depth and sensitivity ,and the
number of winding affect the magnetic field.
DISCRIMINATION ON PI
the time it takes for a target signal to decay can
vary because of the size, shape, and chemical
makeup of the object ,two ways forDiscrimination
 Many PI's rely on the ability of an adjustable
delay whereby the operator can simply adjust the
delay longer to see if an object is a piece of gold
or not
 Another concept used on a PI for discrimination
is to sample during the "on" time of the pulse
Switching
terminals
Arm
Electronic
circuit
enclosure
Coil
•
In designing the metal detector special attention will be
given to:-
Sensitivity and detection depth.
 Endurance.
 Ground noise.
 Target Discrimination
 immunity to electro- magnetic interference .

Applications of metal detectors:1- Can be used in archaeological digging
2- As a compact hand metal detector for security
3- Hardware for Salvage work
4- As a Toy
5-Treasure hunter, for caches of coins, jewelry, weapons
and so on.