History and development about Metal Detector27-05-2019

Towards the end of the 19th century, many scientists and engineers used their growing knowledge of electrical theory in an attempt to devise a machine which would pinpoint metal. The use of such a device to find ore-bearing rocks would give a huge advantage to any miner who employed it. Early machines were crude, used a lot of battery power, and worked only to a very limited degree. In 1874, Parisian inventor Gustave Trouvé developed a hand-held device for locating and extracting metal objects such as bullets from human patients. Inspired by Trouvé, Alexander Graham Bell developed a similar device to attempt to locate a bullet lodged in the chest of American President James Garfield in 1881; the metal detector worked correctly but the attempt was unsuccessful because the metal coil spring bed Garfield was lying on confused the detector.


Modern developments


The modern development of the metal detector began in the 1920s. Gerhard Fischer had developed a system of radio direction-finding, which was to be used for accurate navigation. The system worked extremely well, but Fischer noticed there were anomalies in areas where the terrain contained ore-bearing rocks. He reasoned that if a radio beam could be distorted by metal, then it should be possible to design a machine which would detect metal using a search coil resonating at a radio frequency. In 1925 he applied for, and was granted, the first patent for a metal detector. Although Gerhard Fischer was the first person granted a patent for a metal detector, the first to apply was Shirl Herr, a businessman from Crawfordsville, Indiana. His application for a hand-held Hidden-Metal Detector was filed in February 1924, but not patented until July 1928. Herr assisted Italian leader Benito Mussolini in recovering items remaining from the Emperor Caligula's galleys at the bottom of Lake Nemi, Italy in August 1929. Herr's invention was used by Admiral Richard Byrd's Second Antarctic Expedition in 1933, when it was used to locate objects left behind by earlier explorers. It was effective up to a depth of eight feet. However, it was one Lieutenant Józef Stanis?aw Kosacki, a Polish officer attached to a unit stationed in St Andrews, Fife, Scotland, during the early years of World War II, who refined the design into a practical Polish mine detector. These units were still quite heavy, as they ran on vacuum tubes, and needed separate battery packs.


The design invented by Kosacki was used extensively during the Second Battle of El Alamein when 500 units were shipped to Field Marshal Montgomery to clear the minefields of the retreating Germans, and later used during the Allied invasion of Sicily, the Allied invasion of Italy and the Invasion of Normandy.


As the creation and refinement of the device was a wartime military research 

operation, the knowledge that Kosacki created the first practical metal detector was kept secret for over 50 years.


Further refinements


Many manufacturers of these new devices brought their own ideas to the market. White's Electronics of Oregon began in the 1950s by building a machine called the Oremaster Geiger Counter. Another leader in detector technology was Charles Garrett, who pioneered the BFO (Beat Frequency Oscillator) machine. With the invention and development of the transistor in the 1950s and 1960s, metal detector manufacturers and designers made smaller lighter machines with improved circuitry, running on small battery packs. Companies sprang up all over the United States and Britain to supply the growing demand.


Modern top models are fully computerized, using integrated circuit technology to allow the user to set sensitivity, discrimination, track speed, threshold volume, notch filters, etc., and hold these parameters in memory for future use. Compared to just a decade ago, detectors are lighter, deeper-seeking, use less battery power, and discriminate better.


Larger portable metal detectors are used by archaeologists and treasure hunters to locate metallic items, such as jewelry, coins, bullets, and other various artifacts buried beneath the surface.


Discriminators


The biggest technical change in detectors was the development of the induction-balance system. This system involved two coils that were electrically balanced. When metal was introduced to their vicinity, they would become unbalanced. What allowed detectors to discriminate between metals was the fact that every metal has a different phase response when exposed to alternating current. Scientists had long known of this fact; in time detectors were developed that could selectively detect desirable metals, while ignoring undesirable ones.


Even with discriminators, it was still a challenge to avoid undesirable metals, because some of them have similar phase responses e.g. tinfoil and gold, particularly in alloy form. Thus, improperly tuning out certain metals increased the risk of passing over a valuable find. Another disadvantage of discriminators was that they reduced the sensitivity of the machines.


New coil designs


Coil designers also tried out innovative designs. The original induction balance coil system consisted of two identical coils placed on top of one another. Compass Electronics produced a new design: two coils in a D shape, mounted back-to-back to form a circle. This system was widely used in the 1970s, and both concentric and D type (or widescan as they became known) had their fans. Another development was the invention of detectors which could cancel out the effect of mineralization in the ground. This gave greater depth, but was a non-discriminate mode. It worked best at lower frequencies than those used before, and frequencies of 3 to 20 kHz were found to produce the best results. Many detectors in the 1970s had a switch which enabled the user to switch between the discriminate mode and the non-discriminate mode. Later developments switched electronically between both modes. The development of the induction balance detector would ultimately result in the motion detector, which constantly checked and balanced the background mineralization.


The size of the coil can limit or optimize the size of the target detected. A very small coil can generally pickup smaller targets better than a larger coil. Conversely, a larger coil can usually detect larger objects from farther away, and sometimes sacrifices being able to detect smaller objects (even up close). There are trade-offs for what the  detectorist  is trying to find. Usually a detector user needs to decide which size coil will be used. On some high-performance detectors, sometimes the coil size is fixed (the coil cannot be changed) to optimize the circuitry for detecting smaller objects while still giving good depth on larger objects. Some higher performance metal detectors allow the user to change the coil size, to optimize what the user is searching for; a good example is very tiny gold pieces, usually requiring a smaller coil. Recent coil advancements have had a smaller coil inside a larger coil, and the circuitry is creating special timings between the two coils to optimize smaller and larger object detection simultaneously. Generally when using a metal detector the user should select the coil size based on  ground coverage  desired,  sensitivity  to smaller objects, distance that large objects can be detected, and the amount of  ground noise  that the coil will pickup or be able to  cancel . Smaller coils are sometimes used to focus the detecting search area to be smaller, thereby avoiding  trash  that may be present in a location.


Pulse induction


At the same time, developers were looking at using a different technique in metal detection called pulse induction.[5] Unlike the beat frequency oscillator or the induction balance machines which both used a uniform alternating current at a low frequency, the pulse induction (PI) machine simply magnetized the ground with a relatively powerful, momentary current through a search coil. In the absence of metal, the field decayed at a uniform rate, and the time it took to fall to zero volts could be accurately measured. However, if metal was present when the machine fired, a small eddy current would be induced in the metal, and the time for sensed current decay would be increased. These time differences were minute, but the improvement in electronics made it possible to measure them accurately and identify the presence of metal at a reasonable distance. These new machines had one major advantage: they were mostly impervious to the effects of mineralization, and rings and other jewelry could now be located even under highly mineralized black sand. The addition of computer control and digital signal processing have further improved pulse induction sensors.


The advantages for using a  PI detector  include the ability to  punch through  heavy mineral soil; in some cases the heavy mineral content may even help the PI detector function better. Where a  VLF  detector is usually greatly affected negatively, a  PI  is not.


( Source is from https://en.wikipedia.org/wiki/Metal_detector )