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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING History of metal detectors. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / metal detectors The first mention of metal detectors is found in ancient Chinese documents of the XNUMXnd century BC. BC, which describes the entrance to the chambers of the emperor, made of natural magnetite in the form of an arch, shaped like a horseshoe magnet. Such a magnet has a sufficiently high magnetic force and attracts to itself any iron objects, including weapons brought into the emperor's room. At the end of the XIX century. in different countries, studies were carried out on the then fashionable phenomenon of magnetic induction. In the United States, Alexander Bell, the inventor of the telephone, was actively involved in this. In 1881, US President J. Hartfidd was mortally wounded. The doctors, having heard about Bell's experiments, asked for help finding a bullet in the President's body. However, Bell's device was still very primitive and the bullet could not be found. At the beginning of the XX century. metal detectors were used mainly in the search for minerals, in particular electrically conductive sulfide ores in the USA and England. These devices were quite bulky and therefore mounted on cars. A powerful DC generator, using a huge radiating coil, created an electromagnetic field that penetrated several meters deep into the earth and, reflected from metal and other electrically conductive bodies (some metal ores), was captured by a receiving coil. By changing the characteristics of the secondary field (amplitude and phase shift), it was possible to judge the presence of one or another object in the soil. Subsequently, such powerful installations were banned in the countries that signed the convention on the maximum levels of electromagnetic waves. In 1925, the first arched metal detectors appeared in Germany, which made it possible to detect metal objects that workers secretly carried out of the factory. Around the same time, the German researcher S. Herr developed the principle of magnetic induction balance and invented a metal detector operating on this principle. This idea was immediately picked up by various companies, in particular the Radio Metal Locating Company in the USA, which first launched the production of hand-held metal detectors in the late 20s. The receiving and transmitting coils were separated by 2 meters on a wooden frame to eliminate mutual electronic interference and ensure the induction balance state. Any metal object that appeared in the electromagnetic field of the transmitter disturbed the inductive balance, and a voltage arose in the receiving coil, which amplified and warned the operator about the find in the form of an audible signal. The device worked on 6 radio tubes, was rather bulky, but successfully found pipes at a depth of 3 m. However, in the early 30s, the company went bankrupt. After the Great Depression, there was a construction boom in the United States, and there was a renewed need for tools to find cables and pipes. This prompted Gerhard Fischer, a German engineer who emigrated to the United States in 1923, to develop such devices. In 1937, he received his first patent for a metal detector, which he called a metalloscope, and opened a company for its production. The metalloscope had a more complex circuit, which used 9 lamps. The operating frequency was lowered to kHz, which provided a deep penetration of the electromagnetic field into the ground. In 1938, the scheme of the device was described in detail in a number of popular magazines, thanks to which many radio amateurs made such devices with their own hands. At the same time, another American company, the Goldak Company, produced a similar spaced-coil metal detector called the radioscope. It was the first to solve the problem of detuning from the ground. The same company for the first time patented the design of metal detectors with a round search coil, which almost all modern metal detectors have. It should be noted that the search instruments of the 30s. worked on radio tubes, were made mainly of wood and weighed from 15 to 25 kg. At the end of the 30s. devices with a round search coil, a rod and an electronic unit mounted on the rod began to be predominantly developed. This design was more convenient to use and made it possible to find both small objects and hidden treasures. Beating devices were widely used to search for pipes, which later began to be used to search for mines. The use of mine detectors for treasure hunting began in the late 40s. in the United States, when obsolete equipment began to be sold at military warehouses. They were used at that time mainly to search for treasures and native gold. Only enthusiasts could work with such heavy and uncomfortable devices. And only when in the late 50's. small and light transistor devices appeared, a new hobby began to spread widely throughout America, stimulating the emergence of numerous metal detector companies. These devices worked either on beats or on the principle of induction balance. Their sensitivity was low (10-15 cm for a coin), there was no ground balance and discrimination, but nevertheless they were much more convenient than military mine detectors and made it possible to find many coins, rings and other jewelry. The main manufacturers of metal detectors in the USA in the 60s gg. were G. Fisher, C. Garrett, E. Reis, W. Megan. Significant interest in the search for gold and coins arose in the 70s. in the United States due to a sharp increase in the price of gold. The fact is that in the United States from 1933 to 1974, US citizens were forbidden to have native gold in private hands. The government kept the price of gold artificially low ($35 per ounce) at this point. After the abolition of this law, the price of gold began to rise sharply, reaching in the late 70s. up to $800 per ounce. This served as a new impetus in the search for gold and stimulated the production of metal detectors, which began to be widely used for this purpose. Dozens of new firms have sprung up producing such devices. However, in the fierce competition, only a few firms in the 70s. managed to survive - these are Garrett Electronics, Inc., Fisher Research Laboratory (in 2006 this company was bought by First Texas International), White's Electronics, Inc. (USA) and C-Scoop (England). In the 80s. firms such as Tesoro Electronics, Inc., Ltd. have emerged and are successfully developing, having found their niche. and Teknetics in the US, Minelab in Australia. Small-sized, stable and very sensitive devices appeared.
With the progress in the design of metal detectors, problems such as detuning from metal debris and from electrically conductive soil minerals (iron oxides and salts) were successfully solved, while until the end of the 70s. the two processes could not be done at the same time. The operating frequency of the devices has been reduced from 100 kHz to 1-5 kHz. A scheme for continuous auto-tuning of the device during the search was invented, economical pulsed devices with discrimination appeared, etc. Metal detectors have become more and more complex every year and, unfortunately, heavier. Having listened to the complaints of the search engines, G. Fisher in the early 80s. developed a brand new instrument (1260-X), including automatic ground balancing and automatic discrimination, using new electronic components and a new instrument layout. It was a light, easy to handle and quite effective dynamic metal detector, and soon other companies began to use this principle of operation and the same layout. > The signal processing from the object became more and more accurate every year and improved significantly in the mid-90s. with the advent of computers. The first patent for a computer metal detector was received by Garrett Electronics, Inc., but the first computer device was released to the market by White's Electronics, Inc., the latest modifications of which - Spectrum XLT and DFX - are currently the best in the world. All other leading firms now also produce computer devices. In Russia, until the end of the 80s, there were practically no household metal detectors, although not only individual citizens, but also some departments showed interest in the search for treasures. For this, mine detectors were usually used. Amateur radio enthusiasts assembled devices according to schemes published in magazines, modernized mine detectors, making them more sensitive and stable, but these were single copies. In the early 80s. The best amateur design was, perhaps, the impulse metal detector of V Gorchakov. The first foreign devices appeared on sale in Russia in the late 80s. Modern metal detectors can do almost everything except digging up a find: detect metal, determine which group it belongs to, what its dimensions are, at what depth it lies, establish its exact location, do not react to soil minerals and metal debris. However, these devices still have limitations. They cannot find a coin at a depth of more than 50 cm, show the shape of an object on the screen, determine the chemical composition of the metal, they cannot look only for gold. Metal detectors of the future will undoubtedly have such capabilities. Author: Bulgak L.V.
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