Application of magnetometry for objects identification

PRACE NAUKOWE Akademii im. Jana Długosza w Częstochowie Częstochowie Technika, Informatyka, Inżynieria In Bezpieczeństwa 2014, t. II, s. 475–480 475 http://dx.doi.org/10.16926/tiib.2014.02.43 Jakub Žabenský, Radomír Ščurek VŠB – Technical University of Ostrava ul. Lumírova 13, 700 30 Ostrava – Výškovice, Czechy e-mail: mail: APPLICATION OF MAGNETOMETRY MAGNETOMETRY FOR OBJECTS IDENTIFICATION Abstract. The paper concerns the possibility of application of magnetometry for object identification. One of the basic conditions for successful use of magnetometric methods is the magnetic contrast of solids towards surroundings. Another possible use of magnetometry is for local identification of solids in selected areas, which are situated in the local magnetic field. The magnetometer, which enables to measure up to three vector components of magnetic induciton and specialized software were used in order to perform the research. Two experiments were carried out within the research. Keywords: magnetometry, object identification. identification ZASTOSOWANIE MAGNETOMETRII MAGNETOMETRII DO IDENTYFIKACJI IDENTYFIKA OBIEKTÓW Streszczenie. W pracy zaprezentowano analizy dotyczące dotycz możliwości zastosowania magnetometrii do identyfikacji obiektów. Jednym z podstawowych warunków efektywefekty nego wykorzystania metod magnetometrycznych jest pomiar tła magnetycznego obiekobie tów w odniesieniu do otoczenia. Kolejnym możliwym mo liwym zastosowaniem magnetometrii jest identyfikacja entyfikacja obiektów w lokalnym polu magnetycznym magn w wybranym nym obszarze. W przeprowadzonych badaniach wykorzystano magnetometr umożliwiający umo jący pomiar trzech składowych wektora indukcji magnetycznej oraz wyspecjalizowane oprogramooprogram wanie. W ramach pracy zrealizowano zrealiz dwa eksperymenty naukowe. Słowa kluczowe: magnetometria, identyfikacja obiektów. obiektów 476 Jakub Žabenský, Radomir Ščurek This article points to the possible use of selected properties of solids for their identification and determination of their position if they are situated in the Earth's magnetic field with triaxial magnetometer. One of the basic conditions for succesful use of magnetometric methods is the magnetic contrast of these solids towards surroundings. Another possible use of magnetometry is for local identification of solids in selected areas, which are situated in the local magnetic field. Picture 1. Magnetic induction of different objects [2] Objects containing feromagnetic substances can create static as well as dynamic magnetic field. The intensity of these fields decreases with the distance from the object and vice versa. Because all of these objects are in the Earth's magnetic field, the intensity of the resulting magnetic field measured by magnetometer is given as a vector sum of the custom object fields located in the Earth's magnetic field. This vector sum is influenced by a magnetic background in the defined places. Ferromagnetic object can have permanent magnetization or magnetization induced by the Earth's magnetic field. Permanent magnetization does not depend on the presence of an external magnetic field, therefore if an object rotates, its magnetic field will rotate too. Conversely, the induced magnetization is dependent on the direction and magnitude of the Earth's magnetic field and the magnetic properties of the object. A field created by induced magnetization will Application of magnetometry… 477 not rotate simultaneously with the object. Determining relative additions of these two kinds of magnetization is possible by a series of measurements involving an object’s rotation around its axes (x, y, z). Equipment As was stated above, a magnetometer plays the main role in this article. Magnetometer Vema is designed for vector measurements and oscilloscopic view of the progression of magnetic induction fields as well as creating and recording data files from time and space measurements and also its analysis. Magnetometer enables to measure up to three vector components of magnetic induciton defined by user. The resolving ability of a used magnetometer reaches up to units of nT due to sensors, which are made of magneticaly soft amorphous metal alloy. The constant sampling frequency of magnetometer can be adjusted from 1kHz to 250Hz with sensitivity ≥ 2nT . The following table shows basic parameters of the mentioned device. Table 1. Vema Magnetometer parameters [1] Range ± 100 µT Direct sensitivity ≥ 2,0 nT / 1kHz Average sensitivity ≥ 0,2 nT / 5kHz Sampling frequency 1 kHz Frequency range 0 - 250 Hz Offset drift ± 100 µT (after 15 min) Linearity error 0,5% Temperature range +10°C - +40°C Main power supply 230V / 50Hz / 15VA Battery power supply 12V /500mA Dimensions [mm] 390 x 180 x70 Weight 2,8 kg Number of sensors 3 Interface USB 2.0 478 Jakub Žabenský, Radomir Ščurek Besides the device there is also some software equipment needed. Real time and even post-procesing evaluation of the data obtained from the magnetometer can be performed by QtVema software. The primary function of this software is to visualize the processed data to the user and also to save collected data in memory. Visualisation itself runs in osciloscopic mode on a PC connected to the magnetometer. Identification process The principle of change of magnetic induction can be used in object security, because this change represents a specific state change caused by an object presence at real space (e. g. room). This state can be defined in real space E3 and specific time t by a set of values Ω1 ϵ E3 (t, Bx, By, Bz), which is transformed (by Fourier’s disctrete transformation) and provides us mediated image ψ1 ϵ (t,a,b,c). For practical applications it’s necessary to know the initial or referential state in the defined area [a, b, c] and time. This state is defined by a specific value, which is a function of the specific time, exact position (e. g. GPS) and properties of magnetic background Ω1 ϵ E3 (t1, Ba1, Bb1, Bc1). Then at the specific time values Ωn ϵ E3(tn, Ban ,Bbn, Bcn) are monitored, where the resulting change is given by a difference between Ω1 and Ωn. Experiments The main objective of the first experiment was to study interactions between sources (magnetic, electromagnetic) in the defined area. The dependence of distance and induction of the selected object was monitored with regard to the characteristics of the defined area and furthermore standard electrical sources with a frequency of 50Hz were monitored, especially their first to fifth harmonics and sub-harmonics. The experiment was supposed to confirm or refute the assumption of potential influence by any other sources outside the defined area and also their influence on this area without magnetical shielding. During the first experiment several different items were used – a neodymium ball (5mm in diameter), screw nuts (in sizes M4 and M8). The following characteristics were monitored – course of movement with r (...truncated)