Instrumentation & Measurement Magazine 25-6 - 5

integration processes, the obtained performances suffer from
drift problems over time, and therefore they are characterized
by less stability.
Positioning Based on Alternating
Magnetic Fields
This paper is focused on the use of anchor-based positioning
systems to achieve accurate performance in short-range
applications in indoor working environments. Compared to
outdoor environments where GNSSs represent the de facto
standard of use, indoor environments are characterized by a
higher level of complexity. In such environments, the main
problems to be addressed are due to: reflective materials
that cause multipath propagation; obstacles that weaken the
signal, causing fast fading and Non-Line-of-Sight (NLoS) conditions;
and moving people/objects that affect the reception
of signals [1]. Due to all three problems, the development of
effective and efficient indoor positioning systems today still
represents a scientific and technological challenge.
To address these problems, technology based on low
frequency variable magnetic fields has been used for the development
of some indoor positioning systems. In fact, this
technology does not suffer from the multipath propagation
problem, it guarantees operation in NLoS conditions by
penetrating non-metallic objects, and it is immune to high
frequency interference (typical in indoor environments). In
addition, it guarantees millimeter positioning accuracy. The
state-of-the-art in the literature guarantees relative ease in
generating and measuring magnetic fields. In addition, the
developed systems are compatible to meet the low-cost requirement
thanks to the economy and small size of the devices.
There are several short-range positioning systems based on
magnetic fields developed in recent years that can be divided
into direct current (dc) and alternating current (ac) systems. In
general, for both systems, one or more magnetic field sources,
called transmitters (tx), and one or more magnetic field sensors,
called receivers (rx), can be present. Depending on the
architecture of the developed system, the tag can be a tx or an
rx, and the same goes for anchors. Some positioning systems
use mutually orthogonal coils both for the generation (anchors)
of the magnetic field and for the reception (tag). There
are also systems that use a permanent magnet (tag) for the field
generation and an array of magnetic sensors for reception (anchors).
In magnetic positioning systems, the description of the
magnetic field behavior occurs by using the Biot-Savart law or
with the approximate model based on the magnetic dipole. In
the latter case, for a correct description of the magnetic field, it
is necessary that the distance between the field source and the
estimation point is much greater than the maximum size of the
source. If this condition is not met, this type of approach fails.
In this paper, systems based on ac magnetic fields are used
since they guarantee a greater operating range than systems
based on dc magnetic fields. The developed systems employ
resonant circuits for both the transmission and reception of
the magnetic field. Thanks to the resonance, the system energy
consumption is reduced for the same operating range.
September 2022
The applications described next are for short-range conditions,
due to the physical characteristics of the space propagation of
magnetic fields.
Applications and Results
Magnetic technology has been used for the development of
two positioning systems to be used in industrial and biomedical
contexts. As regards industrial applications, the
experimentation took place in the context of Non-Destructive
Testing (NDT) based on Eddy Current Testing (ECT), in
which a magnetic positioning system can solve the problem of
knowing the probe position during the execution of the ECT,
through low-cost hardware. Regarding biomedical applications,
the aim of the experimentation was to propose a scalable
and low-cost clinical system to monitor the evolution of Parkinson's
disease on the basis of magnetic measurements. The
activities carried out in the selected application fields are described
below.
Probe Localization in Eddy Current Testing
In the NDT context, ECT is a very common technique to check
for defects on metallic materials [5]. To ensure the reliability of
the test result, it is necessary to accurately know the Eddy Current
Probe (ECP) position when scanning the material. Errors
in the knowledge of the ECP position lead to errors in identifying
the position of a possible defect within the material. To
date, there are two different approaches to obtain the ECP position.
Automatic scanning systems can be used to obtain the
probe position with great precision; in this case the probe is
rigidly constrained to the scanning systems. They are characterized
by high costs. Freehand tests can be carried out in
which the operator manually moves the probe to scan the material.
However, in this case the reconstruction of the probe
position during the scan can be complex. In this scenario, a
magnetic positioning system was proposed to estimate the
ECP position in 2D domain [6]. The adopted system is able to
track tag position in 3D scenario, as illustrated in the following
section on Parkinson's disease monitoring. In the ECT application,
we arbitrarily chose to perform only 2D positioning since
the application was based on the assumption that the test was
performed at constant z-value. Nevertheless, whenever the
testing objects were 3D-spaced, it would be easy to extend the
algorithm to such a case.
The proposed system allows for the freehand execution
with real-time feedback of the ECP position, and the increase
in automation remains affordable from the economic point
of view, since the low-cost hardware/software solution proposed
in [6]. The main drawback of the proposed system is
to regards the high accuracy needed to manage the localization
set-up, since the anchors' positions and orientations must
be known with a very high level of accuracy that should be at
least one order of magnitude higher than the required system
positioning accuracy.
In order to verify the reliability of the magnetic positioning
system to provide the ECP position, the set-up shown in Fig.
1 was developed. This set-up provides for the simultaneous
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