Metal Detectors Prepared by : Anwar Abu Khazneh Ola Abu Hijleh 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 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 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

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.