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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Setting up a metal detector. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / metal detectors The maximum efficiency of metal detectors, based on the principle of induction balance, significantly depends on their fine tuning. An incorrectly set device will not detect deep or shallow objects (deep means coin-sized objects at a depth of 30 cm). Moreover, the more powerful the device, the more important the setting is for it. Less powerful devices react to incorrect settings to a lesser extent. Detailed instructions for setting up specific instruments can be found in the manuals that accompany them. Here are given only general remarks on this topic, concerning any metal detectors. Typically, setup includes the following steps:
Threshold setting Adjusting the threshold, or the minimum sound background still audible to the operator, is the first and important step in setting up the instrument. For some modern dynamic metal detectors, the threshold is adjusted only in the "All Metals" static search mode, while in the "Discrimination" mode the device works without a threshold background (silent search). Nevertheless, for the most efficient operation in this mode, it is important to set the required threshold in the "All Metals" mode. To properly adjust the threshold, the coil is held at a distance of 70-90 cm from the ground and metal objects, and with the help of the threshold adjustment knob, a barely audible constant sound background is set, which, in fact, is called the threshold. This background should be minimal and maintained at this level during the entire time the device is in operation. Deep or shallow objects do not give a clear signal, however, thanks to the threshold change, which our ear can easily distinguish, you will already know that there is some kind of metal object under the coil. This is precisely the need for a correct threshold setting. If it becomes louder or disappears, for example, due to temperature fluctuations or a bad pound, for some devices (Imperial 300, TM 808) with a special button you can instantly return it to its original value. Sometimes you have to resort to this button all the time. Therefore, for the convenience of work, a special scheme was developed that allows automatic adjustment of the threshold. This feature is called autotuning. For some devices, you must first adjust the threshold manually, and then using the "Auto-tuning" switch, the set threshold is automatically maintained at a given level during the operation of the device. Auto-tuning is an important component of modern dynamic metal detectors. Inexpensive models of metal detectors usually have a constant average threshold level, which is not always convenient when working in the "All Metals" mode due to the constant sound background. Proper threshold setting is important for manual ground balancing. Ground Balance Pound detuning is necessary to achieve the maximum depth of action of the device, especially in conditions of strong mineralization of the pound. For most modern metal detectors, this operation is performed automatically and continuously as the mineralization changes. Therefore, the information below is relevant to instruments that have a manual ground balance knob. The ultimate goal of ground balancing is to have the same threshold background when the coil is up and down. The first and most important step in ground balancing is to select a ground area free of metal and minerals such as magnetite. The choice of such a site must be carried out in the "All metals" mode with the detuning knob in the middle position. Two types of such handles are used - single-turn with a scale from 1 to 10 and ten-turn. First you need to set the handle in the first case to position 5, in the second - make 5 turns in the opposite direction from the point of difficult rotation. (In general, this handle can rotate in any direction indefinitely, passing 10 revolutions through the friction zone, which can be considered as the starting point). There are two methods of ground balancing. The first, the so-called static method, when after each adjustment of the pound detuning knob, the level of the threshold background is adjusted manually. The other, the dynamic method, provides for automatic threshold adjustment during the detuning process. Let's consider how the detuning from the pound is carried out in both cases. If using the static method, raise the coil above the pound to a height of 35-40 cm. Press and release the threshold adjustment button (or spring-loaded switch), then lower the coil to a distance of about 2 cm from the pound and note the threshold background, which may increase or decrease. If the background volume has increased, turn the pound detuning knob one notch or one turn down. Raise the coil again and press and release the threshold adjustment button. If the Threshold volume decreases the next time the coil is lowered, turn the pound balance knob in the opposite direction half a notch or half a turn in the opposite direction. Then raise the coil and adjust the threshold. By repeating this procedure several times, achieve that the volume of the threshold background would be the same when the coil is lowered and raised. To use the dynamic method, the metal detector must have an automatic threshold setting. The ground balancing process in this case consists of raising and lowering the coil with one hand while turning the ground balance knob with the other hand until the threshold is the same when the coil is up and down. Compared to the static method, this method is more convenient, but not all devices have this capability. These procedures significantly increase the efficiency of the metal detector, although, unfortunately, it should be noted that an ideal 100% ground detuning is possible only theoretically. The presence of electrically conductive minerals in the pound leads to the appearance of secondary signals that mask the main signal from the object, thereby reducing the depth of the device. However, by adjusting the sensitivity, you can improve the performance of the metal detector. Sensitivity adjustment Modern metal detectors have a fairly high sensitivity. So, they feel a coin (in air) at a distance of 30-40 cm. However, the device can detect the same coin in a pound at such a depth only if the coin has lain in it for a long time, has oxidized and the oxides have spread around it in the form of a halo, as if increasing the area of the coin. It is known that the depth of detection of an object in a pound is directly proportional to the area of this object facing the coil. When the area is large, then the secondary signal generated by the object is strong enough. The smaller the object and the deeper it is, the weaker the signal. For example, the device will react to a samovar buried to a depth of 1 m, but no metal detector will be able to detect a coin at such a depth. The fact is that even with a properly performed ground balance, additional secondary signals appear from the minerals surrounding the coin, which the device must separate from the main signal emanating from the object. If we set the sensitivity knob to the maximum value, then the signals from the soil minerals that are harmful to us will also be amplified. Therefore, in highly mineralized ground, the instrument will perform better at a lower sensitivity level, as weak signals from the target will be heard more clearly. Thus, start at the maximum sensitivity level, but then reduce it to a reasonable level if the ground is highly mineralized. Setting the Discrimination Level Discrimination is understood as such a mode of operation of a metal detector, in which there is no reaction of the device to unwanted objects. By changing the level of discrimination, you can ignore certain unwanted objects or groups of such objects. On fig. 22 shows a number of objects in ascending order of their electrical conductivity. Small objects, classified as garbage, have lower electrical conductivity and are relatively easy to retract from, increasing the level of discrimination. Objects with high electrical conductivity (or having a large size) do not allow them to be rebuilt and will be perceived by the device even at the maximum level of discrimination. It should be borne in mind that, when retracting from the tongues of cans, we will lose thin gold rings and other small valuable objects, and very thin gold chains cannot be detected by most metal detectors even at shallow depths.
Rice. 22. A number of objects in ascending order of their electrical conductivity The level of discrimination has a great influence on the depth of detection of objects when working on mineralized soils (Fig. 23).
Curve 1 (good ground) shows that even with a discrimination level of 10, a silver coin can be detected at a depth of up to 28 cm, while most of the debris is ignored by the device. Curve 2 refers to a moderately mineralized pound. The same coin in such a pound can only be found at a depth of up to 15 cm. Curve 3 characterizes a highly mineralized pound. In this case, the loss of depth is maximum. If the discrimination level is set above 5, a large silver coin can only be detected at a depth of up to 5 cm. Recently, devices have appeared with the possibility of selective discrimination, when individual unwanted objects or groups of objects are ignored, both with low electrical conductivity (nails, foil, etc.) and with high electrical conductivity (can tabs). Some metal detectors allow you to adjust the width of the ignoring windows in a fairly wide range. It follows from the above that when working on highly mineralized soils, it is necessary to use a minimum of discrimination. In this case, of course, you will have to dig up more finds related to the category of garbage. At the initial stage of mastering the device, it is recommended not to use discrimination at all and to dig everything in a row. Author: Bulgak L.V.
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Comments on the article: a guest My metal detector has such a problem, it makes an alarming sound and the second light is constantly flickering. What do you recommend? Oleg Set it up.
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