Metal detector
A metal detector (also metal detector and metal probe ) is a device for locating hidden metal parts , pipelines and electrical conductors on land and under water.
History and Development
From 1870 one began to develop devices for the localization of metals. Advantages were promised, especially in mining. The physicist Heinrich Wilhelm Dove invented the induction balance system in the 19th century, which was used in metal detectors a hundred years later as the first system for metal detectors. The first devices required a lot of battery power and only worked to a very limited extent. Alexander Graham Bell tried to use such a device to find a bullet in the chest of US President James A. Garfield in 1881. The attempt was unsuccessful, apparently because the metal coil springs of the bed interfered with.
The modern development of metal detectors began in the 1930s. The technician Gerhard Fischer recognized that radio waves were disturbed or deflected by ore-containing rock and metal parts. Conversely, it should be possible to locate metal with the help of high-frequency waves. In 1937 he acquired the first patent for a metal detector.
Józef Stanisław Kosacki, a Polish officer who was stationed in St Andrews , Scotland in the early years of World War II, succeeded in developing lighter, more powerful equipment. However, these devices also worked with electron tubes and accordingly had large and heavy separate batteries. Nevertheless, the device was very successful and widespread, for example it was used for the first time on a large scale mine search. Manufacturers of new devices brought further ideas to market, for example White's Electronics of Oregon began developing the Oremaster Geiger counter in the 1950s . Another inventor of detector technology was Charles Garrett , who developed the BFO (Beat Frequency Oscillator), i.e. the audio-frequency beat between two LC oscillators , one of which is detuned by metal. With the invention and development of the transistor in the 1950s and 1960s, it was possible to develop smaller and lighter devices with improved circuits and smaller batteries.
The induction balance system brought the greatest technical development. This system consisted of two alternating current-carrying coils, whose inductances were balanced. As soon as metal comes close, they get out of balance, this even allows metals to be distinguished from one another, as each metal results in a different phase shift.
The original induction balance system consisted of two identical coils arranged one above the other. Compass Electronics produced a new design: the two coils were brought into a D-shape and placed next to each other with the straight sections. This system was used in the 1970s. The frequency could be adjusted in order to block out the disturbing influence of ferrous soils in particular.
At the same time, the pulse induction devices were created. In contrast to the beat principle or the induction balance system, pulse induction devices send magnetic pulses into the ground. After a pulse has been sent out, the time that elapses until the pulse has subsided is measured. The decay time increases due to eddy currents that occur in metal present. The devices are less sensitive to the nature of the ground and can reach great depths.
construction
Metal detectors consist of a mostly battery-operated electronic circuit and a search coil through which a low-frequency alternating current flows and whose magnetic field should extend as far as possible. The shape of the coil is either flat (plate or ring shape, double-D without core) or elongated (cylinder coil).
The flat shapes are used to search people or ground surfaces, while the elongated shapes are used to locate objects in cavities such as boreholes or rock crevices.
The electronics and the coil are connected to one another by means of a cable and are usually mounted on a rod with an armrest.
function
Basically, metal detectors can be subdivided according to the underlying measurement method:
- Pulse measurement (English pulse mode ): Here, pulses of a magnetic field are periodically transmitted via the transmitter coil . These generate eddy currents in metallic objects in the vicinity of the coil . The eddy currents in turn cause a signal change in the receiving coil, which can be measured as a voltage immediately after switching off the transmission pulse. Depending on the time course and duration of these eddy current responses to pulses and pulse sequences of different lengths, conclusions can be drawn about different metals and the size of the metallic objects. The signal evaluation takes place in the time domain .
- Alternating current excitation or English continuous wave or CW mode : Here, a continuous alternating current is generated in a transmitter coil. There are 2 procedures:
- Attenuation analysis: transmission field in the frequency range of a few 10 kHz. The received signal is analyzed in terms of amplitude and phase position in the receiving coil . The transmission principle is a magnetically coupled system, similar to a transformer . Metallic objects, but also electrically conductive liquids ( electrolytes ), lead to a change in the received amplitude and phase position in relation to the transmitted signal. These two independent parameters enable different materials and metallic objects to be distinguished. Furthermore, through the use of different transmission frequencies, which can also be transmitted simultaneously, the search objects can be further classified.
- many devices (self-made and leisure) only have one search coil and work in the spectral range . The coil is part of an LC oscillator and has a natural resonance frequency of a few 100 kHz. The frequency changes resulting from the field displacement and / or the permeability of the metal objects to be searched for are made audible by comparison with a reference oscillator as a beat in headphones. The frequency-increasing field displacement and the frequency-reducing permeability can cancel each other out, so that iron parts of a certain shape cannot be found.
The different signal responses for different metals and substances and the possibility of detection are related to their substance constants. The primary influencing factors for differentiation are magnetic permeability and electrical conductivity . With these constants, non-metals differ significantly from metals. The mobility of the charge carriers is also important.
The signal change is evaluated electronically in all measuring methods and made visible on an optical scale (e.g. various LEDs ) or made audible by an acoustic signal transmitter above a certain threshold value. With industrially used metal detectors, for example in the food industry for quality assurance and avoidance of metal splinters in food, the signal evaluation is used for the automatic control of the production plant.
application
Metal detectors have many areas of application:
- In airports and other sensitive areas for identity control .
- To find the position of metallic lines and pipes in the floor and walls.
- In the food and pharmaceutical industries to find metal fragments in the products.
- During the treasure hunt to find metallic objects such as coins, jewelry etc. (see also probe users )
- In archeology , the metal detector provides valuable services for initial orientation on excavation sites of the metal-bearing epochs, large-scale planning (prospecting) and the rescue of monuments on large construction sites (emergency salvage). However, their proper use requires the highest level of discipline, so that situations where they were found are not prematurely destroyed without documentation. Because of this risk, many archaeologists are skeptical of the use of metal detectors in excavations.
- When disposing of ordnance to track down land mines , duds, ammunition and the like.
- In the area of machine protection: Examination of tree trunks before sawing or of plastic granulate before processing in the extruder as well as during the manufacture of fleece and film.
Inductive proximity switches work on the same principle as metal detectors, but are comparatively small and therefore have short switching distances . They are used in automation technology for position determination and as limit switches.
