D
Journal of Mechanics Engineering and Automation 5 (2015) 411-414
doi: 10.17265/2159-5275/2015.07.005
DAVID
PUBLISHING
Modeling of Earth Magnetic Field Disturbances Induced
by Ferromagnetic Objects
Michal Nowicki1 and Roman Szewczyk2
1. Industrial Research Institute for Automations and Measurements PIAP, Warsaw 02-486, Poland
2. Institute of Metrology and Biomedical Engineering, Warsaw University of Technology, Warsaw 02-525, Poland
Abstract: This paper presents the methodology and results of modelling of Earth magnetic field disturbances induced by ferromagnetic
objects. The modeling was carried out using the finite elements calculations, and the results were compared with the measurements.
The special test stand, consisting of magnetovision scanner and Helmholtz coils, has been used. The measurement system is able to
measure the distribution of planar magnetic induction vectors, and to differentiate the sources of measured disturbances.
Key words: Magnetovision, magnetic anomalies, FEM (finite element method).
1. Introduction
The paper presents the investigation of influence of
the high magnetic permeability objects on the natural
Earth’s magnetic field. The ferromagnetic materials
influence surrounding magnetic field because of own
remanent magnetization, which in effect causes them to
act like a magnetic dipole, and because high
permeability object magnetizes itself in Earths
relatively weak field. Thus, it creates distortions in
otherwise homogenous Earth’s natural magnetic field.
There is an ongoing research to connect said
distortions with high magnetic permeability objects
presence and localization [1], which in turn would
allow for detection and localization of nearly
demagnetized ferromagnetic objects. Existing passive
gradiometer systems are designed for deep-level search
of relatively big targets, such as unexploded aircraft
bombs. They are used for magnetic imaging, using data
logging and GPS (global positioning system), but they
have very low resolution [2, 3]. Therefore, a
high-resolution scanning system is designed for search
of small targets from small distance, such as potentially
dangerous objects in baggage.
Corresponding author: Michal Nowicki, Ms. Eng.,
research fields: magnetic measurements, and magnetic
materials. E-mail: [email protected].
There is an open question however, about the
magnitude of the magnetic permeability related
distortion effect of various ferromagnetic objects,
especially in comparison with the remanent
magnetization. To investigate this subject, special test
stand was constructed, consisting of magnetovision
scanner and high precision tri-axial Helmholtz coils.
The Helmholtz coils were used to set the reference
magnetic field for the measurement—simulate the
Earths field at various angles in relation to the sample.
Moreover, the paper presents an application of
magnetostatic calculations based on the free
open-source Elmer software [5] for the modeling of the
disturbances caused by small, ferromagnetic objects.
The overall effect would be very hard to calculate using
analytical methods; therefore, the FEM (finite element
method) was implemented. Furthermore, comparison
between modeling results and actual measurements is
shown.
2. Measurement Setup
In order to measure the permeability and remanence
related distortion effect, special test stand was
developed, consisting of magnetovision scanner and
high precision tri-axial Helmholtz coils. The
magnetovision scanner can measure the planar
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Modeling of Earth Magnetic Field Disturbances Induced by Ferromagnetic Objects
magnetic field vector values distribution. For the
measurement, it uses tri-axial high sensitivity
Honeywell magnetoresistive sensor. Thin-film
magnetoresistive sensors are the most suitable for
magnetic imaging. They exhibit high sensitivity and
have small size, typically 1 × 1 mm2, or less [4]. The
schematic of the setup is presented in Fig. 1. The
Helmholtz coils were used to set the reference magnetic
field for the measurement under reduction of the natural
background field. First, the demagnetized cylindrical
steel sample was situated in the center of the Helmholtz
coils. Then, the Earth’s natural magnetic field was
compensated, and the magnetovision scanning was
performed to measure the remanent magnetization
distribution. Then Helmholtz coils were used to
simulate natural field acting in the X, Y, and Z axis of
the sample, and the scans of magnetic field distributions
were performed. By the subsequent data processing, the
planar distributions of magnetic field distortions related
to materials permeability and sample remanence were
calculated.
3. Modeling Methodology
To evaluate the results, and create the possibility of
said effect modeling in objects of various shapes and
composition, method of calculation of constant
magnetic field distortions was implemented, using the
finite element calculations method utilizing the
specialized open-source Elmer software [5].
First, the shape of the sample, measurement plane
and background were described in the .geo format. The
‘imaging’ planes were made of a neutral cube around
the sample and a cylinder with a volume of constant
magnetic flux distribution, served as the constant
magnetic field standard, analogically to the Helmholtz
coils in the test stand. Next, the basic geometrical
shapes were processed by open-source Netgen 5.1
software, and meshed with very fine net [6]. Typical
number of interconnected calculation nodes was
200,000, which allowed for relatively high modeling
resolution. The mesh was then exported and saved
Fig. 1 Schematic block diagram of the measurement
system. Sample—the investigated object is placed inside the
Helmholtz coils. The magnetovision scanning system is
following the measurement lines in the plane directly above
the sample, within the coils.
in .gmsh format. In order to prepare the meshes for
further processing, and remove unnecessary nodes,
artifacts and false objects, additional cleaning of them
was made using the Elmergrid autoclean function [7].
Finished meshes were loaded into the Elmer
software, together with .sif files defining the bodies
and borders, material types, relative magnetic
permeability values and active solvers and equations.
Elmer is mainly oriented on solving partial differential
equations. WhitneyAV solver included in the
MagnetoDynamics module of the software was
utilized, which allowed for calculation of the magnetic
flux density and magnetic field strength in every node
of the modeling area. The solver is based on the
classical Maxwell equations. The ElmerGUI VTK
postprocessor allows for many different kinds of
visualization of the results, including colorbar surfaces,
vectors and isosurfaces.
4. Experimental Results
Simulation results of various magnetovision images
were obtained, for different geometry samples,
different sizes and magnetic permeabilities. In the Fig.
3, the typical result is shown, for a steel cylinder
sample of 70 mm diameter, on a 200 × 200 mm2
measurement plane. The relative position of the sample
is shown in Fig. 2c. The external constant field was set
in the Z axis of the sample, perpendicular to the sample
Modeling of Earth Magnetic Field Disturbances Induced by Ferromagnetic Objects
Fig. 2
(a) Background cylinder mesh, defining the
simulation area and boundary conditions; (b) imaging cube
consisting of orthogonal measurement planes; and (c)
relative position of the sample in the modeling, the same as
in the real test stand measurements.
and measurement plane, to simulate natural
geomagnetic field. Absolute values of magnetic flux
density are in μT. The total distortion effect is shown.
Fig.4 is the actual measurement of distortion caused by
real-world sample. The distortion value is strongly
dependent on remanent magnetization, and magnetic
domains distribution causes the non-symmetrical shape
of the obtained image. Other possible causes are
material defects, nonlinearities and anisotropy.
Nonetheless, the distortion distributions are strikingly
similar.
In Fig. 5, the modeling results of the same sample, in
same position are shown. In this simulation, only the
magnetic permeability related effect is considered, the
sample was treated as a perfectly demagnetized one.
413
Fig. 3 Result of magnetic disturbance modeling for the
constant magnetic field in the perpendicular (Z) axis of the
sample (results presented in μT).
Fig. 4 Result of sample measurement in homogenous
magnetic field of 50 μT, in the Z axis (perpendicular to
sample and measurement plane).
Positive and negative disturbances of constant external
magnetic field are clearly seen. Fig. 6 is the same
situation, but measured IRL. The remanence-related
magnetic flux density distribution in near-zero external
field is measured, and then in the set field of 50 μT.
Data processing which has been done in Matlab allows
the separation of the permeability-related effect. The
disturbances amplitude and distribution is similar to the
modeled results, but again non-symmetrical due to the
more complex nature of real objects.
Fig. 5
Result of permeability-related magnetic
disturbance modeling for the constant magnetic field in the
Z axis of the sample (results presented in μT).
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Modeling of Earth Magnetic Field Disturbances Induced by Ferromagnetic Objects
the developed test stand. They validate the usefulness
of this modeling method, based on the open-source
software for finite elements magnetostatic calculations.
The visible differences are caused by the
inhomogenous magnetization of the real world sample.
However, on the basis of the simulated and measured
results, the presence and location of the object can be
determined, which is very useful from practical point
of view.
References
[1]
Fig. 6 Result of sample measurement in homogenous
magnetic field of 50 μT. Background and remanence field
removed to show the permeability-related effect.
[2]
5. Conclusions
The experimental stand for planar measurements of
the vector distributions of weak magnetic fields
distortions and for differentiating its sources was
developed. Moreover, new methodology of the
modeling and its results was presented. The developed
test stand allows also to visualize the distribution of the
absolute values of magnetic flux density vectors. The
values and directions of the magnetic flux density
vectors in different measurement points have been
determined. Experimental investigation allowed to
measure of magnetic permeability and remanence
distortion effect. The modeling results are similar to the
real-world magnetic disturbances images obtained on
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