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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING The principle of operation of the metal detector. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / metal detectors Useful Definitions Dynamic mode of operation provides for continuous movement of the search head (coil). The reaction of the device will be observed only when the head is passed over the metal. If the head is stopped over an object, the reaction will disappear. Static operation - such a mode in which the reaction of the device will be maintained while the head is above the object. It doesn't matter if it moves or not. Pseudo-static mode of operation - dynamic mode with very slow threshold adjustment. That is, if the head is held over an object for a long time, the device will adjust to this object and the reaction will disappear. When the head moves away from the object, a reverse adjustment of the same duration will begin. In computerized devices, the threshold adjustment processes are controlled by a microprocessor, so the adjustment speeds when approaching and moving away from the object can be different. In conventional devices with pseudostatics, the RETUNE button is used to quickly restore the threshold when the coil moves away from the object. VCO mode - the dependence of not only the volume of sound, but also its tone (frequency) on the size and depth of the object. Sharpens the reaction of the human ear to slight changes in the signal received from the object. Threshold (threshold tone) - a weak sound tone heard in the speaker of the device or headphones in the absence of metal objects near the head. Threshold tone is available both for devices that have a search mode for all metals, and devices with static search modes. By this tone, one can judge the performance of the metal detector and the change in the mineralization of the soil. In most devices, the threshold tone is adjustable. Rejection - the absence of a sound signal (ignoring) for a certain type of objects. Basic principles of building a metal detector: advantages and disadvantages There are several principles by which metal detectors are built. Here are the main three. BFO (Beat Frequency Oscillator - beat generator, dependent oscillator) - addition of vibrations of two high-frequency generators and selection of the difference between their frequencies, which lies in the sound range. If a metal object appears near the search coil, the frequency of the signal of one of the generators changes, and with it the pitch of the audible sound. That is, they work on the basis of determining small changes in the inductance of the search coil under the influence of iron objects. They are characterized by low sensitivity. TR / IB (Transmit-Receive / Induction Balance - transmission, reception and induction balance) - two coils are installed in the sensor, transmitting and receiving. The presence of metal changes the inductive coupling between them, which affects the received signal. PI (Pulse Induction) - transmission is carried out by pulses, the shape and amplitude of the received signals depend on the presence of metal objects near the detector coils. On the contrary, pulse detectors have many advantages:
When they work, magnetic pulses are used that can induce current in all metal objects that fall into a magnetic field. Between pulses, the receiver receives a response, which is amplified and processed by the electronics. With their undeniable advantages, they also have disadvantages:
Since the mid-90s, many pulsed metal detectors have been equipped with a discriminator. Balanced Induction Metal Detectors became standard detectors for general use. They have two coils in the search head, one of which induces an alternating magnetic field. The other coil is positioned so that the field around it is normally balanced and there is no electrical signal at its output. In fact, there is a so-called residual signal in the receiving coil, which is not due to the ideal design. Metal objects that approach the coils change the configuration of this field and unbalance the system. As a result, a signal appears at the output of the receiving coil. This signal can be amplified and inform the operator about the find. Modern metal detectors using this principle have powerful electronics that process the signal and give the operator a lot of additional information, for example:
Each principle has specific advantages and disadvantages. Therefore, in expensive and complex detectors that can determine not only the presence, but also the type of metal, all three are combined. Functioning of the metal detector When the metal detector is turned on, an electromagnetic field is created in the search coil, which propagates into the environment:
Eddy currents arise on the surface of metals that have fallen into the area of action of the search coil under the influence of an electromagnetic field. These eddy currents create their own counter electromagnetic fields, leading to a decrease in the power of the electromagnetic field generated by the search coil. This is fixed by the electronic circuit of the device. In addition, the 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 received information and signals the detection of metal. Eddy currents form on the surface of any metallic objects or electrically conductive minerals. The determination of the metal in an object is based on the measurement of the electrical conductivity of the object. Frequency ranges of metal detectors In the technical literature on metal detectors, many technical terms are used. So, the designation VLF (Very Low Frequency) means a very low operating frequency of the metal detector. The ability of a metal detector to discriminate (recognize) the nature of the find depends on the operating frequency of the device. At high frequencies, the skin effect is enhanced, and the quality of discrimination deteriorates significantly. Therefore, at first (70s, early 80s), metal detector manufacturers used very low frequencies of about 2 kHz. This led to specific problems:
Modern metal detectors have a wide spread in operating frequency, this is due to the specifics of their application, as well as for engineering and design considerations. Most often, the frequency range extends from 6 to 20 kHz, but sometimes lower. In this frequency range, devices discriminate well against targets and there are no serious problems with the design of the coils. Devices for searching for gold use higher frequencies - up to 15-20 kHz and above. This is also due to the fact that at these frequencies the sensitivity to very small objects improves, for example, to gold nuggets, which often have a meager size and low weight. In recent years, to improve the depth and quality of discrimination, multi-frequency search has been used, which under certain conditions gives advantages. With the advent of cheap microprocessors, this method received a strong impetus in its development. But I would like to make two important remarks about multi-frequency search:
When searching for large objects, the size of a liter jar or more, in conditions of slightly mineralized and slightly littered soil, these metal detectors are very convenient. Since the mid-70s, they have practically fallen into disuse. Today, a typical representative of this type of metal detectors is Fisher's Gemini-XNUMX. Some manufacturers of metal detectors used this designation as an addition to the term VLF, apparently in order to once again emphasize that the device is based on the principle of induction balance. Adjustment of the earth compensation system Soil mineralization - the presence in the soil of electrically conductive salts and minerals, as well as minerals and rocks containing iron. It leads to a violation of the correct operation of the devices. It is eliminated by the introduction of additional circuit solutions and modes. In the process of working on the ground, the nature of the soil can change, and, consequently, its mineralization. This will require an adjustment of the earth compensation system. This is done both manually and automatically, if the detector has such an operating mode. It received the designation Ground Track. This mode is not recommended for high sensitivity settings. The term VCO is used to denote the sound mode of the metal detector, in which, depending on the size and depth of the object, not only the volume of the sound, but also its tone changes. This sharpens the hearing perception of the slightest change in the sound signal. Modern high-end metal detectors have a rich service that allows an experienced operator to conduct a fruitful search with the least amount of time. If the search engine additionally has an idea about the principle of operation of the metal detector and its real capabilities, this brings additional dividends. About the sensitivity of metal detectors To obtain high sensitivity, the total gain of the entire receiving path of the metal detector can be tens of decibels. A reckless increase in sensitivity leads to a deterioration in the stable operation of the device, thus turning the metal detector into a useless search tool. Sensitivity is the main, but not the main advantage of a modern metal detector. Understanding of this fact comes with experience and becomes obvious after the searcher gets to the area heavily littered with metal junk. Here already and high sensitivity is not a joy! Sometimes the pursuit of supersensitivity in some individuals degenerates into a disease when a person focuses on every extra centimeter of depth indicated in the brochures. Firms-sellers skillfully play on this fact! And finally, the detection depth, as a rule, is given in the air, i.e. for idealized conditions, but in the ground the indicated object at the declared depth, most likely, will not be able to be detected. Author: Dubrovsky S.L.
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