Metal detector

A metal detector (also metal detector and metal detector ) is a device for locating hidden metal parts, pipes and electrical conductors on land and under water.

History and Development

With the beginning of the 20th century efforts were made to develop a device for locating metals. Benefits promised you mainly in mining. The physicist Heinrich Wilhelm Dove invented in the 19th century, the induction balance system, which was used in metal detectors a hundred years later the first system for metal detectors. The first devices were very simple, you need a lot of battery power, and worked only to a very limited extent. Alexander Graham Bell tried to use such a device to find a bullet in the chest of American President James Garfield in 1881. The attempt was unsuccessful, apparently because the metal coil springs of the bed disturbed. The modern development of the metal detector began in the 1930s. The technician Gerhard Fischer realized that radio waves were disturbed by erzhaltiges rock. It should thus be inversely possible to locate by means of high frequency waves metal. In 1937 he obtained the first patent for a metal detector. Jozef Stanislaw Kosacki, a Polish officer who was stationed at a unit in St. Andrews, Fife, Scotland in the early years of World War II, managed the same improvement. However, these devices were very heavy and required vacuum tubes and separate batteries. Manufacturers of new devices brought their own ideas to the market soon, so began to develop the Oremaster Geiger counter about White's Electronics of Oregon in the 1950s. Another inventor in detector technology was Charles Garrett, who developed the BFO (Beat Frequency Oscillator). With the invention and development of the transistor in the 1950s and 1960s, it was possible to develop ever smaller and lighter devices with improved circuitry and smaller batteries.

The biggest technical development brought the induction balance system. This system consisted of two coils that are electrically balanced. Once metal is introduced into their environment, they come from the " equilibrium", this allows metals to " discriminate ", so to distinguish from each other, since each metal produces a different phase response when AC power is suspended.

The initial balance inductor system consisted of two identical coils, superimposed on each other. Electronics compass produced new design: the two coils were placed in a D-shape, to form an opposing circuit. This system has been used in the 1970s. In order to exclude the mineralization of the soil, the frequencies could be adjusted.

At the same time the pulse induction device formed. In contrast to the beat - frequency oscillator or the balance induction devices, the asynchronous pulse device sends a high voltage pulse signal to the ground. With the transmission of a pulse while the very minimum time difference is measured, which is produced when the pulse encounters metal; while the voltage increases.

Construction

Metal detectors consist of a mostly battery powered electronic circuit and a carrying low-frequency alternating current search coil, the magnetic field should extend as far as possible. The shape of the coil is to either flat ( plate or ring shape, double-D without core ) or stretched ( solenoid ).

The flat forms are used for screening of persons or floor surfaces, while serving the stretched forms for the localization of objects in cavities such as boreholes or rock crevices.

Electronics and the coil are connected to each other by means of a cable, and generally mounted on a linkage arm rest.

Function

Basically can be divided metal detectors after the underlying measurement method:

• Pulse measurement (english pulse mode): It will be transmitted via the transmitter coil periodic pulses of a magnetic field. These produce metallic objects in the vicinity of the coil, eddy currents. The eddy currents in turn cause in the receiving coil a signal change that is measurable as a voltage immediately after turning off the transmit pulse. Depending on the Timeline and duration of the eddy current responses to different lengths of pulses and pulse sequences can be used to determine different metals and on the size of the metallic objects. The signal evaluation takes place mostly in the time domain.

• AC Current Measurement (English continuous wave or CW mode): This is a continuous mostly low-frequency current in a coil is generated in the frequency range of a few 10 kHz. In the reception coil, the reception signal according to amplitude and phase position is analyzed in the spectral domain. The transfer type is a magnetically coupled system, similar to a transformer. By metallic objects, as well as by electrically conductive fluids ( electrolytes), this results in a change of the received amplitude and phase based on the transmission signal. These two independent parameters are used to distinguish different materials and metallic objects. Further, by the use of different transmission frequencies, which can be transmitted at the same time, carried a further classification of the search objects. Devices that operate on the principle of alternating current measurement, may, but need not operate by resonant method. For simple devices for leisure use the search coil is part of an oscillator (a few 100 kHz). Frequency changes are then made ​​audible with a reference oscillator as beats and applied directly to a headset.

The different signal responses at different metals and materials and the possibility of detection is related to the material constants. Primary factors to differentiate the magnetic permeability and the electrical conductivity. Nonmetals differ in these material constants of metals significantly. In addition, the mobility of the charge carriers is important.

The signal change is processed electronically for all measurement methods and on an optical scale ( for example, various LEDs) visible or audible by an acoustic signal transmitter above a certain threshold. Industrially used metal, for example in the food industry, for quality assurance and avoid metal pieces in food, the signal processing for the automatic control of the production plant is used.

Application

Metal detectors have many applications:

• In airports and other sensitive areas to control people.
• To find the position of metallic cables and pipes in the floor and walls.
• In the food industry and pharmaceutical industry to find metal fragments in the products.
• In the treasure hunt to locate metallic objects, such as coins, jewelry, etc. (see also detectorists )
• In archeology, the metal detector provides a valuable service in the Initial guidance on burial sites of the metal leading epochs of large-area planning ( prospecting ) and the rescue of archaeological monuments on large construction sites ( Notbergungen ). However, their proper use requires rigorous discipline, lest rashly locality situations are destroyed undocumented. Because of this risk, many archaeologists use the metal detector on excavations are skeptical.
• The explosive ordnance disposal for the detection of landmines, unexploded ordnance, munitions and the like.
• In the area of ​​machine safety: examining logs before sawing or plastic granules before processing in the extruder and in the nonwoven and film production.

Inductive proximity switches operate on the same principle as metal detectors, however, are relatively small and therefore have lower operating distances. They are used in automation technology to determine the position and as limit switches.