How metal detectors work.
Mark Rowan & William Lahr (translated by A. Nikitsky)
How metal detectors work.
INTRODUCTION.
Metal detectors (MD) are wonderful machines. Many people who use MDs are full of enthusiasm, extolling the features of their favorites before heading out to hunt for treasure. Those of us who professionally design and make these tools listen carefully to what people say about their work in the field, as this is a vital way to find out how well we are doing and what needs to be improved. Sometimes communicating with customers can be difficult. We literally speak different languages.
The purpose of this article is to try to at least partially break down the existing language barrier. And to try to explain all those «mysteries» that sometimes occur when using the device ineptly.
Do you need to know how a detector works in order to use it effectively? Of course not (provided you read the instructions carefully — translator's note). Can such knowledge allow you to use your device's capabilities more fully in the future? Surely yes, but only with some diligence and training. After all, the best detector works as well as how well it is used.
Ultra Low Frequencies. Transmitter-Receiver Circuit
TRANSMITTER
Inside the search frame of a metal detector (also called a search head, coil, antenna) is a wound wire called a transmitting coil. Electric current flowing through it creates an electromagnetic field. The direction of the current changes several thousand times per second to the opposite, and the characteristic «operating frequency» tells how many times per second the current moves clockwise and counterclockwise.
When the current flows in one direction, a magnetic field is created, directed toward the ground; when the current direction changes to the opposite, the magnetic field will be directed away from the ground (like the south and north poles of a school magnet). In any metal (or even electrically conductive) object that is nearby, under the influence of such a changing magnetic field, electric currents will arise, in many ways similar to those that arise in the winding of a generator rotating in a constant magnetic field. The induced current, in turn, will create its own magnetic field, with a direction opposite to the magnetic field of the transmitter.
RECEIVER
Inside the frame there is another «receiving» coil, located in such a way as to maximally neutralize the influence of the transmitting coil, for which special methods are used. But the field from a metal object that is nearby will induce a current in the receiving coil, which can be amplified and processed by electronics, having previously been separated from the more powerful signal of the transmitter.
The total received signal usually appears with some delay relative to the transmitted signal. This delay is caused by the fact that conducting materials have properties that resist both the flow of electric current (resistance) and the change in the magnitude of the current already flowing in them (inductance). We call this apparent delay the «phase shift». The largest phase shift will be produced by objects that are mostly inductive — these are large, thick objects made of excellent conductors such as gold, silver and copper. Less phase shift is typical for objects that are inherently resistive — these are smaller, thinner objects or objects made of materials with worse conductivity.
Materials that conduct electricity poorly or not at all can also produce a strong signal in the receiver. Such materials are called ferromagnetic. Ferromagnetic objects become strongly magnetized when placed in an external field (for example, a paper clip that is attached to a magnet brought near). The signal in the receiver will show little or no phase shift. Many types of soil contain tiny grains of iron-bearing minerals, which will be detected as ferromagnetic by the detector. Metal castings (forged nails, for example) and steel objects (beer caps) will exhibit both ferromagnetic and conductive properties.
It should also be noted that the circuits described here are «inductive balance» detectors, sometimes called VLF circuits — extremely low frequency (below 30 kHz). Currently, this is the most popular technology, which also includes low-frequency circuits (30…300 kHz).
DISCRIMINATION
Since the signal received from any metal object will exhibit its characteristic phase shift, it is possible to classify different types of objects and distinguish them. For example, a silver coin gives a much greater phase shift than an aluminum button, so you can set up the detector so that it will beep in the first case and remain silent in the second, or identify the object on the display, or deflect the needle of the microammeter. The process of recognizing metal objects is called discrimination (recognition, separation). The simplest form of discrimination allows the device to beep when the frame is passed over an object whose phase shift exceeds the average (adjustable) value. Unfortunately, devices with this type of discriminator will not respond to some coins and most jewelry if the discrimination level is set high enough (to ignore common aluminum junk like buttons or drug caps).
A more useful circuit is the so-called notch discriminator. This type of circuit responds to objects within a certain range (for example, the «nickels and rings» range) and will not respond to phase shifts above that range (buttons, drug caps) or below it (iron, foil). More advanced detectors of this type can be configured so that for each of several ranges it will either respond to or, on the contrary, ignore phase shift signals within it. For example, the White's Spectrum XLT device allows you to program 191 different ranges.
Metal detectors can be equipped with various information reading devices: a digital display, indication on a pointer device, and others that help identify an object. We call this feature VID (visual discrimination indicator) and its main function is to give the operator the opportunity to make an informed decision about whether to start excavating, without relying only on the sound signal. But most, if not all MDs equipped with VID also have a sound recognition system.
The type of metal object can be predicted from the ratio of its inductance to its own resistivity. For a given transmitter frequency, this ratio can be calculated from the delay (phase shift) of the signal arriving from the object. An electronic circuit called a phase detector can measure this phase delay. Typically, two such phase demodulators are used, with the peak values of the signal they measure offset from each other by 1/4 of the transmitter wavelength, or 90 degrees. We call these two channels X and Y , respectively. A third demodulating channel, called G, can be tuned so that its response to any signal with a constant phase shift relative to the transmitter pulses (such as soil) can be reduced to zero, regardless of the amplitude of this signal. This is necessary in order to separate the two components of the signal — the response from the soil and from the object — and determine the most likely type of object.
Some metal detectors use a microprocessor to process these three channels and determine the most likely type of object. The ratio of the X and Y channel readings, regardless of the G () channel value, is a number. We can find this ratio with good resolution — better than 500 to 1 over the entire range of materials encountered, from ferrite to pure silver. The signal from iron objects is orientation sensitive, so the numerical response can change dramatically as the frame moves over them. Graphic displays that plot the X/Y ratio on the horizontal axis and the amplitude of the received signal on the vertical are very useful for sorting out metal trash from more valuable objects. We call this type of display a «sigmagraph». (SigmaGraph TM)(see figure below)
GROUND BALANCE
(ground balance)
As mentioned earlier, most soils contain iron. They can also have conductive properties due to the presence of salts dissolved in the groundwater. Therefore, the signal received by the detector from the soil can be 1000 times stronger than the signal from a metal object buried deep in the ground. Fortunately, the phase shift of the received signal from the soil remains fairly constant over a certain area of the Earth's surface. The detector can be designed so that even when the ground signal changes greatly — for example, when raising and lowering the frame, or when the operator walks along an embankment or over a pit — the detector readings will remain unchanged. Such a detector is said to be «ground-tuned». Good ground tuning makes it possible to determine with great accuracy both the location of an object and to estimate its depth. If you select the «all metals» mode — without discrimination of signals by phase shift — good ground offset is especially important.
In its simplest form, ground balancing looks like this: the operator raises and lowers the frame of the metal detector, turning the tuning knob and achieving equality of indicator readings. Although this method is quite effective, it can seem tedious, and for some users, quite difficult. More expensive MD models perform ground balancing automatically, usually in two steps: the first — with the head raised, and the second — with the head lowered. The «smartest» devices will adjust constantly, so that you will not even notice it when switching from one type of soil to another. This is the so-called «tracking ground balance». Good detectors with this function allow you to tune in once and spend the rest of the day searching without additional adjustments. But be warned: most MDs that are sold under the sign «automatic» or «tracking ground balance», are actually just set by the manufacturer to some fixed level of ground balance. It's a bit like if you were to have your gas pedal welded to the floor of your car at «medium gas» and were told that your car had a modern «cruise control» system. J
DYNAMIC AND STATIC MODES
(motion/non-motion modes)
Although the ground signal may be much stronger than the object signal, the ground signal still tends to remain constant or change very smoothly as the loop moves. On the other hand, the object signal increases sharply to a peak value and then decreases as the loop passes over it. This opens up opportunities to use the technique of object recognition not by the amplitude of the received signal, but by the rate of its change. This mode of MD operation is called «motion mode». The most important example of using this principle is motion discrimination. If we want to isolate useful signals sufficient for identifying an object, it is not enough to just detach from the ground. We need to look at the object from two different angles, much like we do when triangulating to determine the distance by selecting more than one observation point. Having detach from the ground at one point, we get a certain combination of the ground and object signals at the other. And the dynamic mode is used to minimize this residual ground signal. Currently, all discriminators and VID detectors require constant frame movement for effective metal detection. This is not such a big problem, since you still need to move during the search.
If you find an object in the dynamic discrimination mode, you will probably want to determine its location more precisely, so as not to dig in vain. If your detector has a depth gauge, you will also want to measure the depth. To accurately determine the position and depth, use the «all metal» mode. Discrimination is not needed here, so you do not need to move the frame, except for those movements that bring the frame to the exact center of the object. To be more precise, the speed with which you move the frame in this mode is not important. Therefore, the «all metal» mode is often called «non-motion mode» (also called «normal mode» or «DC mode» (D.C. mode)).
There are a few things that might confuse you in the brochures of the detectors. Some detectors have a «self adjusting threshold» (SAT) feature that slowly increases and decreases the audio output, providing a quiet but audible «threshold» sound. This helps smooth out changes caused by changing soil types or poor ground balance. «Self adjusting threshold» can be fast or slow depending on the detector type and its settings, but frankly, SAT is very similar to a dynamic mode. That's why you may read ads for «detectors that have a true non-motion mode», which essentially means an «all metal» mode without SAT. Another thing that can sometimes be confusing: some discriminators allow you to set the threshold so that the discriminator starts responding to all metals. In other words, it is a discriminator that does not discriminate. This is something different from the «all metal» mode described above. This mode is often called «zero disk».
MICROPROCESSOR CONTROL
A microprocessor is a complex electronic circuit that performs all the logical, arithmetic, and control functions necessary to build a computer. A sequence of instructions stored in the processor's memory is called a program and is executed by the processor sequentially, one after another, at a speed of up to several million actions per second.
The use of microprocessors in modern MDs opens up possibilities that were unimaginable a few years ago. In the past, adding new useful functions to an MD meant adding new buttons and switches. At some point, the size, cost, and complexity of operating such a device went beyond reasonable limits. A microprocessor, a liquid crystal display, and a simple keyboard became the solution to the problem. A virtually unlimited number of new functions can be built into the device without changing its appearance. Only the built-in menu system is supplemented, and by following the instructions on the screen, almost anyone can figure it out and configure the device according to their wishes. Thus, the same MD can be configured for any operator.
But what if you don't want to bother with all these settings? This is where the genius of microprocessor control comes into play — you don't even have to. When you turn on the device, all parameters are set to some preset values, so a beginner or casual user may not even know about all the additional capabilities of the device. And what's even better — by simply going through the menu, you can select modes for coin search, general viewing, archaeological search, etc. — and the microprocessor will perform all the necessary settings, as this has been verified by many years of experience of veterans of the search business.
Let's add to this that powerful software support has improved the sound functions of the devices for determining the required metals, and images on the LCD monitor in various forms speed up and simplify the operator's work.
CONCLUSIONS ON VLF SCHEMES
Although VLF devices have been manufactured for over 10 years, improvements in performance are constantly being made. More and more «smart» and easier to use devices are being developed. Rest assured that as long as there are undiscovered treasures, new and improved devices will be developed, no matter how perfect the existing ones may seem.
Pulse Induction
TRANSMITTER
The device search coil or loop MD with pulse induction is very simple compared to VLF devices. A single coil with wound wire is used for both transmission and reception.
The transmitter circuit consists of a simple electronic switch that short-circuits this coil for a short time to the power battery. The resistance of the coil is very low, so a current of several amperes can flow through the coil. Although the current is high, the time it flows is very short. The electronic switch sends a current pulse to the coil, then cuts it off and then turns on again to send the next pulse. The duty cycle, the ratio of the time during which the current is transmitted to the time when the current is off, is usually about 4%. This protects the transmitter and coil from overheating and reduces battery drain.
The pulse repetition rate (transmitter frequency) of a typical MD with pulse induction is about 100 hertz. Different MD models use frequencies from 22 hertz to several kilohertz. The lower the transmission frequency, the greater the emitted power.
At lower frequencies, greater depth and sensitivity of detection of objects made of silver are achieved, but at the same time, sensitivity to nickel and gold alloys decreases. Such devices have a slow reaction, therefore, they require very slow movement of the frame.
Higher frequencies increase sensitivity to nickel and gold alloys, but are less sensitive to silver. They may not penetrate as deeply as lower frequencies for silver, but you can move the frame more quickly. This allows you to search a larger area in a given period of time and also such devices are more sensitive to the main beach finds — gold items.
The pulse induction meter loop with which we began this section consists of a single coil of wire that serves both as a transmitter and a receiver. The transmitter acts like an ignition coil in a car. Each pulse of current in the transmitter coil creates a magnetic field. When the current is interrupted, the magnetic field around the coil suddenly disappears, but at that moment a high-amplitude voltage pulse of opposite polarity appears at the coil terminals. This voltage spike is called the counter electromotive force, or back EMF. In a car, this is the high voltage that ignites the spark in the spark plug. In our case, the pulse induction meter loop has a lower amplitude, typically 100 to 130 volts peak. The pulse is very short, about 30 millionths of a second (30 microseconds). It is called the «reflected pulse.»
RECEIVER
The decay time of this electrical pulse depends on the electrical resistance of the coil with the wire. Complete absence of resistance, or, on the contrary, its very high value will cause the pulse to “ring”. This is similar to throwing a rubber ball on a very hard surface, where it bounces many times before finally settling down. With sufficient electrical resistance, the pulse decay time is shortened and the reflected pulse is “smoothed out”. This is similar to throwing a rubber ball into a pillow. We are interested in our ball one ……..time, which in the case of a rubber ball can be described as throwing it on a carpet. The coil of a detector with pulse induction is said to be critically damped when the reflected pulse quickly fades to zero without “ringing”. Excessive or insufficient suppression will introduce instability into the operation and mask well-conducting metals such as gold and reduce the detection depth.
When a metal object is close to the search coil, it stores some of the pulse energy, which will cause the process of attenuation of this pulse to zero to be delayed. The change in the width of the reflected pulse is measured and signals the presence of a metal object.
In order to isolate the signal of such an object, we must measure the part of the pulse where it falls to zero (tail). At the input of the receiver from the coil there is a resistor and a limiting diode circuit, which cut the voltage of the input pulse to 1 volt, so as not to overload the input of the circuit. The signal in the receiver consists of a pulse from the transmitter and a reflected pulse. Typically, the gain of the receiver is 60 decibels. This means that the area where the reflected signal falls to zero can be increased by 1000 times.
Strobe circuit
The amplified signal from the receiver is fed to a circuit that measures the time it takes for the voltage to fall to zero. The reflected pulse is converted into a pulse train. When a metal object approaches the coil, the shape of the transmitter pulse will not change, but the reflected pulse will become slightly longer. An increase in the pulse tail by just a few millionths of a second (microseconds) is enough to detect the presence of metal under the coil. Pulses (strobes) synchronized with the start of the transmitter pulse are superimposed on this reflected pulse, and a series of strobes is obtained at the output of the electronic circuit, the number of which is proportional to the length of the pulse tail.
The most sensitive pulse is located as close as possible to the end of the tail, where the voltage is very close to zero. Typically, this is the time region of about 20 microseconds after the transmitter is turned off and the reflected pulse begins. Unfortunately, this is also the region where the operation of the PI MD becomes unstable. For this reason, most PI MD models continue to generate strobe pulses for another 30-40 microseconds after the reflected pulse has completely died out.
Integrator
Next, the strobed signal must be converted into a DC voltage. This is done by an integrator circuit that averages the pulse sequence and converts it into a corresponding voltage, which increases when the object is close to the frame and decreases when the object is moving away. The voltage is further amplified and controls the sound control circuit.
The period of time during which the integrator collects incoming strobes, the integrator time constant (ITC), determines how quickly the MD responds to a metal object. A large ITC (on the order of seconds) has the advantage of reducing noise and simplifying detector setup, but requires very slow movement of the loop, since the object can be missed with rapid movement. A small ITC (on the order of tenths of a second) responds to the target faster, which allows faster movement of the loop, but noise immunity and stability of operation are deteriorated.
DISCRIMINATION (recognition)
MDs with pulse induction are not capable of the same degree of discrimination as VLF devices
By measuring the increasing period of time between the end of the transmitter pulse and the point at which the reflected pulse decays to zero (delay), it is possible to filter out objects made of certain metals. Aluminum foil is the most susceptible to this characteristic, followed by small nickel coins, buttons and gold. Some coins can be detected by a very long pulse tail, but iron is NOT detected this way.
Many attempts have been made to create a PI detector capable of detecting iron, but all have had very limited success. Although iron produces a long «tail», silver and copper have the same characteristics. Such a long delay has a bad effect on depth determination. The mineral content of the soil will also lengthen the reflected pulse, changing the point at which the object is detected or rejected. If the integration constant is adjusted so that a gold ring is not detected in air, the same ring may «glow» in salt-saturated soil. Thus, salt-saturated soil changes everything that relates to the delay time and discrimination of the PI detector.
GROUND BALANCE
Ground balance is very critical for VLF devices, but not for PI detectors. On average, the soil does not store any significant amount of energy from the search coil and usually does not give any signal itself. The soil will not mask the signal from a buried object and, on the contrary, soil mineralization slightly lengthens the signal proportionally to the increase in the depth of the object. In relation to PI detectors, the term «automatic ground balance» is often used — they usually do not react to excessive soil mineralization and do not require external adjustment for different types of soil.
An exception is one of the most unpleasant components of the soil — magnetite (Fe3O4), or magnetic iron oxide. It overloads the input coils of VLF detectors, greatly reducing their sensitivity. Detectors with AI will work, but may show false targets if the coil is brought too close to the ground. You can minimize this harmful effect by lengthening the delay time between the end of the transmitter pulse and the beginning of the strobe. By adjusting this time constant, you can tune out interference caused by soil mineralization.
AUTOMATIC AND MANUAL SETUP
Most PI detectors have manual tuning. This means that the operator must turn the tuning until a clicking or buzzing sound is heard in the headphones. If the soil in the search area changes from ….. to neutral sand or from dry soil to sea water, then tuning is necessary. If this is not done, you can lose detection depth and miss some objects. Manual tuning is very difficult when using a short PVI, so many devices with manual tuning have a long PVI and require a slow movement of the frame.
There is no problem with using a pulse induction detector for underwater searches, since the search coil is not moved quickly. When used in the surf zone, the coil will be in and out of the water, and under such conditions, using devices with manual settings can be very frustrating, since you will have to constantly adjust the response threshold. Some operators in this case immediately adjust the device slightly below the response threshold. But this can lead to a decrease in detection depth, when the soil characteristics change.
Automatic tuning (SAT — self adjusting threshold) gives a significant advantage when searching in and over salt water or on soil with a high salt content. It allows you to use the detector at maximum sensitivity without constant adjustment. This improves stability, noise immunity and allows you to use a higher gain. MDs with pulse induction do not emit strong negative signals like VLF devices. Therefore, they do not go off scale in pits with minerals. It is necessary to continuously move the frame of a metal detector equipped with an automatic tuning system, so if you stop the frame, the setting is lost or the device stops responding.
Audio control
Pulse induction detector sounder circuits fall into two categories: variable frequency and variable volume. Variable frequency circuits, based on a voltage-controlled oscillator, are good for detecting small objects because the change in frequency is easier to detect by ear than the change in volume, especially at low volume levels, especially for devices with manual threshold adjustment. However, the sound, similar to a fire siren, quickly becomes tiring, and some people are unable to distinguish high tones. One good option is mechanical vibration, which was originally used for underwater vehicles. Such a device produces clicking sounds and vibrations that increase to a buzzing sound when an object is detected. The signals of such a mechanical device are easy to recognize and they are not drowned out by the air supply system.
Many people prefer a more traditional audio tone with increasing volume rather than frequency. Such audio control systems work well in devices with a fast frame movement, i.e. in devices with automatic tuning, while they sound similar to devices with VLF.
Conclusions on PI detectors
These are specialized instruments. They are of little use for coin hunting in urban conditions, since they cannot filter out iron (ferrous) trash. They can be used for archaeological searches in rural areas where there is no large amounts of iron trash. They are designed for searches at maximum depth in extreme conditions, such as sea coasts or places where the ground is highly mineralized. Such PI detectors show excellent results in such conditions and are generally comparable to VLF devices, especially in their ability to tune out such soils and «pierce» them to maximum depth.