About metal detectors.
About metal detectors
Metal detectors are electronic induction devices that allow you to detect metal objects in a neutral or weakly conductive environment, i.e. in soil, water, walls, wood, under clothing and in luggage, in food products, in the human and animal body, etc. The rapid development of microelectronics has made these devices compact, reliable and very «intelligent». The scope of application of metal detectors has expanded significantly and, in addition to purely professional and military applications, has begun to cover the field of entertainment, which includes “treasure hunting” in the broad sense of the expression.
The main areas of application of metal detectors, determining their design, functions, cost, etc. are the following:
For the military metal detectorThis is, first of all, a mine detector. The ability of the device to distinguish metals is not required here. Until recently, high sensitivity was not required either, but with the advent of plastic mines, the situation has changed: in a plastic or ceramic mine, there is one irreplaceable metal part left — a small spring in the detonator. Only a highly sensitive device can detect it. In addition, the mine detector must be all-weather, all-season, extremely easy to operate and extremely reliable.
In security structures and forensicsThe metal detector is used more widely now than anywhere else. Anyone has encountered such devices when entering a bank, airport or night club. First of all, these are «gates», through which even small metal objects can be detected, and small metal detectors for personal inspection. They easily and with high accuracy localize a suspicious object. Internal troops guarding places of detention are always equipped with such devices. In the West, terrorists often practice sending letters with explosive devices planted in them, mainly to the media. Trying to protect themselves from this, large institutions have special devices for checking incoming mail.
No self-respecting food production company(not in Russia), be it a confectionery factory or a sausage shop, cannot do without a metal detector. Until recently, in our country, the discovery of foreign objects in food products threatened the enterprise — the manufacturer — with nothing more than accusations of negligence and a note in the local newspaper under the heading «Shame on the bad guys» …. Now this can lead to legal proceedings, and in countries with more developed consumer market structures, getting, say, a nail into a sausage is a sure way to bankruptcy of the meat processing plant (its competitors will take care to publicize this fact) and legal liability of the staff. Naturally, it is better to spend a few hundred dollars on a device than to pay legal costs later.
Irreplaceable metal detectors in constructionand during repair work. Think about how to do without this device if you need to draw up a reconstruction project for an old building for which there are no plans for the location of beams and other supporting structures (you can't just hammer away at an architectural monument anywhere); and if you need to trace how a pipeline or electric cable runs in the ground (you can't dig up everything around); or you just want to drill a hole in the wall of your house with an electric drill, then not knowing the location of the wiring can cost you your life. A metal detector will help save your life or save you from unnecessary work.
In the process of processing wood, especially from suburban forests, nails and other metal objects are found in the trunks. The only way to save the saw or avoid damage to other equipment is to check the wood with a metal detector. A special frame installed in front of the circular saw will automatically stop the conveyor when metal is detected in the wood.
In mining, especially for native gold, a metal detector is simply irreplaceable. Many gold mines in America, Australia and other countries experienced a rebirth when, with the help of metal detectors, the productivity of prospectors increased tenfold. A similar «revolution» is still awaiting Russian gold miners: metal detectors are just beginning to appear in the vast expanses of the Russian Trans-Urals.
To the Archaeologista metal detector will help to determine the most promising place for detailed excavations or will provide an opportunity to extract interesting finds where continuous excavations are simply impossible for various reasons.
Treasure hunters and treasure hunters work side by side with archaeologists. There is nothing that can compare with the joy of discovery — whether it is the first ancient coin found, a ring lost centuries ago, or a hoard of ancient jewelry. Indeed, treasure hunting captivates everyone who has picked up a metal detector. It was during the development of equipment for treasure hunters that devices achieved the greatest perfection in sensitivity and discrimination (filtering out unwanted finds). The search for lost objects by people, a kind of «home archeology», has reached great proportions in America. There are clubs of searchers and collectors, for example, belt buckles or cufflinks. Special magazines are published. Dozens of companies producing equipment for treasure hunters operate abroad.
This guide contains a brief description of metal detectors from the most famous companies, used specifically for archaeological and treasure hunting purposes.
1. Operating principle and main groups of metal detectors
What is a metal detector, and how does it manage to distinguish metals?
A metal detector is an electronic device that detects the presence of metal without contacting it (due to the emission of radio waves and the capture of secondary signals), and, having detected it, informs the operator of this fact (with a sound signal, movement of the arrow, etc.).
When the device is turned on, an electromagnetic field is created in the search head, which spreads into the surrounding environment, be it earth, stone, water, wood, air. On the surface of metals that fall within the search coil's range, so-called eddy currents arise under the influence of the electromagnetic field. These eddy currents create their own counter electromagnetic fields, leading to a decrease in the power of the electromagnetic field created by the search coil, which is recorded by the electronic circuit of the device. In addition, this secondary field distorts the configuration of the main field, which is also captured by the device. The electronic circuit of the metal detector processes the information received and signals the detection of metal. Eddy currents are formed on the surface of any metal objects or conductive minerals. The determination of metal in an object is based on measuring the specific electrical conductivity of the object.
The difference between cheap and expensive models is only in the methods of radio wave emission and the methods of capturing, processing and interpreting secondary signals. A more expensive device can determine with a certain degree of probability the type of metal detected before its extraction, determine the depth of its occurrence, can be tuned out from soil minerals, and also have many different additional functions that increase the productivity and efficiency of the search, which are absent in cheap devices.
The following main approaches to the construction of metal detector circuitry are distinguished in the literature:
1) BFO — beat frequency oscillation. The measured parameter is the frequency of the LC generator that includes the search head coil. The frequency is compared with the reference frequency and the resulting difference beat frequency is output to the sound indicator. The circuitry of the devices is quite simple, the coil does not require precision execution. The operating frequency is 40 — 500 kHz. The sensitivity of BFO devices is low with low stability of operation and a weak ability to tune out from wet and mineralized soil. The BFO method was used in serial foreign devices in the 60-70s. Currently, this method is popular with radio amateurs and is found in inexpensive devices from Russian manufacturers. This also includes devices with direct frequency measurement, which are well implemented on microprocessors.
2) TR/VLF — transmitter-reciver/very low frequency. The search head is formed by two coils located in the same plane and balanced so that when a signal is applied to the transmitting coil, a minimal signal is present at the outputs of the receiving coil. The transmitting coil is often included in the LC generator circuit. The measured parameter is the signal amplitude on the receiving coil and the phase shift between the transmitted and received sinusoidal signals. VLF is a variation of this method, when the operating frequency is reduced to 2 — 10 kHz.
VLF — the method allows to build highly sensitive devices with good discrimination of metals due to the analysis of phase characteristics. The circuitry of the devices is quite complex, the coils require precision balancing. Most serial devices, including computerized ones, are now built using this method. Discrimination of objects and tuning from the ground in such devices is done relatively simply with the help of phase-shifting circuits.
The TR principle (or its variation TR/VLF) involves analyzing the phase characteristics of the signal, so they all easily distinguish between ferrous and non-ferrous metals, and are tuned out from trash and soil. These devices have high sensitivity and resolution, which depends on the diameter of the head — the larger the head, the deeper the detection, but the more difficult it is to search for small objects. The term TR discrimination usually refers to the recognition of metals in statics.
3) RF — radio frequency (radio frequency) — a high-frequency version of TR, where the transmitting and receiving coils do not form a flat transformer, but are spaced apart and located perpendicular to each other. The receiving coil receives a signal reflected from a metal surface, emitted by the transmitting coil. This method is used in deep-sea devices and is characterized by insensitivity to small objects and lack of discrimination of metals.
4) PI — pulse induction. In devices of this type, the search head coil is not part of the oscillatory circuit. A pulse signal is fed to it from the trigger generator. The analyzed parameter is the end time of the transient process (the position of the trailing edge of the voltage pulse). There are no special requirements for the coil design. Distinctive features of this method are: low operating pulse repetition frequency (5-600 Hz), high energy consumption, insensitivity to soil, poor metal recognition. The PI method is often used in underwater devices to reduce the influence of water.
5) OR — off resonance (resonance breakdown). The analyzed parameter is the amplitude of the signal on the coil of the oscillatory circuit, tuned close to resonance with the signal supplied to it from the generator. The appearance of metal in the coil field causes either reaching resonance or moving away from it, depending on the type of metal, which leads to an increase or decrease in the amplitude of oscillations on the coil. This method, like BFO, was developed by radio amateurs, but no information was found about its use in serial treasure hunting devices.
As the design of the device becomes more complex and its cost increases, the ability of the device to recognize a metal object without digging it improves. With a difference in cost by several times, the sensitivity of the detectors increases insignificantly (most often it is 20 — 45 cm for coins and about 1 — 2.0 m for large finds). However, even complex devices equipped with processors can give a very approximate conclusion about the metal and the depth of the find.