Principles of operation and characteristics of metal detectors

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Principles of operation and characteristics of metal detectors

simple metal detector works by “Frequency beating .” It consists of two generators set of very close frequencies. One of them is with stable frequency , and the frequency of the other is determined by the parameters of the seeker head. If the seeker head coil close to a metal object , inductance and changes , thus changing the frequency of the generator. Since the signals of the two oscillators with similar frequencies , when mixed to yield a signal with a beat frequency ( in the audio range) , the difference between the two frequencies. This signal could be amplified and fed to the headphones.
When the detector is closer to the metal , depending on whether it is magnetic or not , the tone in the headset has changed. In one case, the tone is lowered , and in the other – is increased .
This type of detectors are of simple construction , and in contrast, are distinguished by the number of disadvantages. Difficult to maintain constant frequencies of the two generators . The distance between the coil and the ground ( the Earth’s ) influence the frequency variation . This avoids having approximately coil sensor placed Faraday shield ( screen). Detector is also affected by salt water, wet grass, or change the pitch.

Another type of metal detectors work on the principle of eddy currents . When approaching a metal object ‘s magnetic field seeker head ( bobbin ) of LC oscillator induced in the surface of the object eddy currents , which leads to its heating . This is related to the absorption of energy , which , in turn , causes a decrease of the amplitude of the generator. The detected voltage of the generator trigger comparator control low frequency generator and loudspeaker. There are diagrams of generators where amplitude is increased and decreased for ferrous metals. This type of detector while being slightly better than the previous version have almost .

Metal detectors are built on the principle of ” induction balance” . Seeker head of these devices is connected to two coils that overlap and are very precisely balanced . One coil is connected to a generator , and its magnetic field crosses the second coil so that, following the exact balance in it induces a very low voltage , so there is no signal at the amplifier output device and therefore the headphones .
If the seeker head closer to the metal, the balance between the two coils is being violated, and the second coil voltage increases, amplified by the AC amplifier is detected , amplified by differential DC amplifier which, by comparator  generator of sound frequency and is applied to this type of detectors . change the distance between the coil and the ground affect machine.

All here described metal detectors have a significant disadvantage that they are influenced by the soil, not ground balance makes them Inaccurate when searching for metal objects in the ground.

Principles of operation and characteristics of metal detectors

Modern metal detectors work on the principle of induction balance phase detected. Their detector is controlled by an appropriate offset of the rotary fazo block signal generator , thereby avoiding the influence of the soil. There are considerably more complex solutions that avoid the influence of ore , slag and other less conductive materials .

” TRANSMITTER – RECEIVER ” is another principle in the construction of metal detectors.
In this case the generator is connected to the transmitting coil to a receiving amplifier bobina.Dvete at an angle close to 90 degrees ( right angle ), it is selected so that, when in the vicinity of the unit will metallic object of the amplifier input voltage is bit, and after amplification and detection can not  low frequency generator. When the detector closer to the metal due to the transformer effect in the receiving coil to induce a signal. It is amplified and fed to the headphones or a meter . Using the concepts of transmitter and receiver is quite arbitrary, since the device does not work with radio waves and the electro magnetic field.
Metal detectors , built on the principle of ” impulse INDUCTION ” have only one coil, which, unlike all previous detectors will need shielding . When it is submitted to tension it creates a magnetic field proportional to the number of turns and the strength of the current, pro running through them. With the exclusion of the voltage transition begins a process in which the voltage drops to zero , and then increases with the opposite polarity . Reverse voltage correctly ” damping ” coil decreases certain law .
If the coil is to the metal during the transients , it either magnetised or produces eddy currents , depending on whether it is magnetic or not. In both cases, the opposite e.d.n. decreases to zero for a – long. Once the signal from the coil is stronger, it is taken strobe pulse when the signal value becomes close to 0V. Gating voltage is applied to an integrator which provides an output voltage proportional to the size of the metal object and inversely proportional to the distance to it . This voltage controlled oscillator whose signal is fed to the headphones.

Here inventoried equipment , all without exception work on the principle of eddy currents , their relative separation is only for the purpose of accessing explanation.

Below we will briefly examine some methods of study relying on other principles.

Electric resistance method to study the practice is to measure the resistance of the soil. Use an ohmmeter to the AC can not be used widely ommetri therefore that all models perform measurement with constant current, it can cause a false indication in the case that between one of the electrodes and the ground or between two types of soil found between electrodes generating potential difference ( voltage source ) . Study itself represents resistance measurement between any two of the electrode 4 , in the event that some of the measurements in a gap of the other , this means that between the two electrodes there is an anomaly . If the resistance is greater than the other measurements, this means that between the two electrodes has a region with a higher resistance than that of the soil nprimer stones, if the resistance is less than the other measurements, this means that between the two electrodes has a region with a lower resistance than that of the soil nprimer metal, ore , sand , salt or wet soil. Thus able to locate only large objects not much depth , suitable for search of underground facilities. There are variants working with 50, 100 electrode connected to a computer which controls the measurement , analyzes it , and presents it in graphic form ,  shown on the monitor as a stain which can roughly judge the size and depth of the object.

GEO RADAR ( scanner) broadcasts through an antenna similar to a reference television aerials pulse lasts from about 0.5 to about 10 nano seconds, and this electromagnetic wave propagates into the earth from 2 to 10 times slower than the speed of light , depending on the type soil or rock when on the way she move from one soil to another soil type or object , part of it as the difference between the two substances is reflected and returned , this signal is received by the second antenna same as the first , and amplified by a stroboscopic unit is stretched in time (e.g., 500 nano seconds, it 100 000 nano seconds) . This signal is converted to digital and adopted by the computer. Both antennas are most frequently formed as sheina which is towed over the terrain to be investigated. Adopted by computer signals are displayed on the screen as colored curve depending on the signal level. If sheinata crawl on the ground until the computer screen fills with the received signals as arrange them side by side on the monitor receives a color picture  ( another layer of soil type , cavity or an object ) appears colored stain on location it can if you know the dielectric constant of the soil studied to determine the approximate depth of the object. Geo radar is suitable for searching caves , tunnels , underground buildings and relatively large objects .

MAGNITO metric method of study is a measurement of the Earth’s magnetic field with precision magnetometer . Close to metal objects magnetometer account variation of the Earth’s magnetic field. This method is suitable for searching metal objects with a large mass in the range of colors and cubic meters is used to search for shipwrecks and ore deposits .

Gravitational method of study is a measurement of gravity with precision gravitometar . Near the solid metal objects gravitometara increased . This method as well as magneto metric is suitable for searching metal objects with a large mass in the range of colors and cubic meters is used to search for mineral deposits and many major sites .

SEISMIC method used  study of seismic waves (these are oscillations mechanical wave with a frequency of a few hertz and below 1 hertz ) to travel large distances in the soil. seismic waves are used mostly micro burst and a huge electro magnetic models dildo. Radiated seismic wave propagates very large distances at the way she move from one soil to another soil type or object , part of it depending on the differences between the two substances is reflected back. On the surface are investigated microphones that capture the reflected wave after filtration and computer processing to obtain information about the Earth’s layers and the presence of anomalies in the study area. Seismic waves due to their low frequency have a longer wavelength resulting in less permissive power , this means that they may be localized only much larger anomalies in the ground. Used for geological prospecting , locating underground caves and large objects.

Devices operating on the principle of nuclear magnetic resonance explained with simple words are popular generator that bobina by an electromagnetic field with a frequency equal to the resonance frequency of the atoms of the desired product . This leads to excite them , and then turn off the electro magnetic field excited atoms continue for some time to emit secondary electromagnetic field of its resonant frequency, it is adopted by the coil , amplifies and after computer processing indicates the presence of a substance. This method of study has nothing to do with the so-called remote locators Omnitron …., Electroscope ….., Lectra Search and more. .

Study with ultrasound is not applicable to the search for hidden items in the ground proradi fact that ultrasound waves subside very quickly in porous materials such as different types of soil. They are useful in liquid and solid materials , therefore, can be used to search for holes and caverns in rock but in soil. As there are many peculiarities in their use , one of them so as not to subside quickly (in order to achieve a – depth ) should izpozva ultrasound at a frequency below 100 kHz.

In archeology is used to study X-rays. A disadvantage of this method is that they are not reflected on the ground can be evaluated not only relatively large volumes which is possible on one side to put  of gamma rays , while the other side fotoplaka packed so that it can to no access to the light , only the gamma rays most frequently black paper. After a suitable duration exposure and development is obtained x-ray of the object .