The research activity of Leonardo Sandrolini is mainly in the
fields of
- Electromagnetic compatibility
(a) methods and techniques for the calculation and measurement of
shielding effectiveness
(b) electromagnetic couplings within metallic enclosures
(c) models to predict electromagnetic interference in switching
converters
(d) electromagnetic interferences in railway systems
- Electromagnetic characterization of materials
(a) Study and extraction of the complex permittivity of dispersive
materials
(b) Study of the electrical properties of adhesive materials for
restorative dentistry
- Electrical aspects of energetics
(a) electrical characterization of renewable sources of electrical
energy
(b) wireless transmission of electrical energy with nonradiative
techniques
- Analysis of electromagnetic fields
- Electromagnetic compatibility
(a) methods and techniques for the calculation and measurement of
shielding effectiveness
The research activity in this field has been started by Leonardo
Sandrolini during the Ph.D. period and concerned the development
and setup of an analytical methodology for the calculation of the
shielding effectiveness of multilayered shields for low-frequency
magnetic fields. This methodology allows one to obtain a good
estimate of the shielding effectiveness of multilayered shields and
a simple shield model at the same time. In fact, numerical
techniques cannot be applied easily to multilayered configurations,
especially when the number of layers is large and the layer
thickness is much larger than the skin depth. The shield is
represented by means of a transmission matrix which expresses
relationships between continuous field components at the two layer
surfaces and allows the multilayered shield to be represented
through multiplying the transmission matrices of all layers of the
shield. The analytical method, based on the solution of the
diffusion equation for the magnetic field (quasistatic
approximation) was validated for frequencies higher than the
industrial one. The highest admissible frequency (as a function of
the distance between source and field points) for the
quasimagnetostatic approximation was found from the comparison
between the solutions obtained with this methodology and those from
the wave equation. The methodology was then applied to magnetic
fields generated by nonsinusoidal currents. The development of an
analytical method made it possibile to implement this methodology
in an optimization procedure for multilayered shields. The
objectives can be either to obtain the minimum shield volume with
the constraint of keeping the magnetic induction field below a
threshold, or to ensure a minimum shielding effectiveness. Two
optimization procedure have been developed, the former based on a
heuristic algorithm, the latter on an evolutionary approach that
makes use of the particle swarm optimization algorithm. The latter
has been found particularly effective as the number of optimization
variables for a multilayered shield is not fixed.
More recently, a research activity devoted to the methods and
techniques of measurement for the shielding effectiveness has been
started. Polymeric conductive materials and conductive woven and
nonwoven textiles have been considered.
(b) electromagnetic couplings within metallic enclosures
A research activity was started to develop a methodology to predict
electromagnetic coupling inside metallic enclosures. This activity
was born from a joint collaboration with the George Green Institute
for Electromagnetic Research, University of Nottingham, United
Kingdom (directed by Prof. Christopoulos) through a formal
Agreement with the aim to develop joint research programmes.
The methodology is based on the analogy between a mono-modal
rectangular waveguide and a transmission line. This analogy
originates from the equality between the power carried by a
propagating mode in the waveguide and that carried by the
equivalent transmission line. The basic radiating structures
considered are wires and loops, that can be modelled as electric
monopoles/dipoles and magnetic dipoles, respectively. The position
of the radiators can be totally arbitrary within the metallic
enclosure. The complete coupling can be represented in terms
of equivalent circuits for the radiating structures and
transmission line circuits for the multi-modal waveguide
propagation path. Inside a metallic enclosure (modelled as a
waveguide short-circuited at both ends) an electric dipole
(monopole) will couple via a mutual capacitance with the waveguide
modes and a magnetic dipole will couple via a mutual inductance.
Dependent current sources effect the mutual coupling of the wire
(monopole) to the waveguide via a propagating mode. In the same
way, dependent voltage sources effect the mutual coupling of the
loop (magnetic dipole) to the waveguide via a propagating mode. The
section of the transmission line between source and victim is
modelled as a two-port network. The equivalent circuit can be
easily extended to include higher order modes than the dominant
one, TE10. For each additional mode, an extra analogous
transmission line is added to the circuit (with its associated
current or voltage sources and impedances). Nodal analysis can then
be employed to solve the equivalent circuit. A number of
experimental tests has been carried out to validate the methodology
and the calculation procedures.
This methodology can be extended to treat coupling between electric
monopoles/dipoles and magnetic dipoles inside a metallic enclosure
in presence of conducting planes. Conducting planes within the
enclosure can be modelled by means of a transmission line analogy.
In fact, the way an electric field is scattered by a conducting
plane inside an enclosure is similar to the scattering of a voltage
wave by a shunt impedance across a transmission line. The ultimate
aim of the methodology is to implement a calculation code to
predict the electromagnetic coupling between printed circuit boards
and other radiating structures, such as interconnecting cables,
inside metallic enclosures.
(c) models to predict electromagnetic interference in switching
converters
The research activity in this field is focused to develop
mathematical models for the calculation of radiated electromagnetic
fields. Different methodologies to predict the radiated
electromagnetic fields from straight interconnect cables carrying
high-frequency currents have been studied. Measurements on an
experimental setup have been carried out to verify the accuracy of
the analytical models in different configurations where a pair of
cables located at various distances from a conducting ground plane,
parallel to the plane of the cables, has been examined. Similarly,
radiated emissions from PCB lands were studied; the p.u.l.
parameters of the PCB lands were preliminarily calculated.
An alternative approach to describe The electromagnetic field
coupling at low frequency (9 kHz-30MHz) between electromagnetic
interference (EMI) sources of switched-mode power supplies (SMPS)
and a victim circuit makes use of two-port networks represented by
short-circuit admittance and open-circuit impedance matrices. The
currents in the primary and secondary loops of a SMPS (source
circuit) create a time-varying magnetic flux which links the victim
circuit where a resulting voltage is induced. This coupling is
known as inductive or magnetic field coupling. Furthermore, the
voltage between a heatsink and the reference ground of the SMPS
(source circuit) creates a time-varying electric field which
extends through space and thus generates a displacement current,
which may terminate in the victim circuit. This coupling is called
capacitive or electric field coupling. The key point of this
research activity is to calculate the relevant open-circuit
impedance and short-circuit admittance parameters. The mutual
impedance between source and victim circuit is calculated by the
induced EMF method.
(d) electromagnetic interferences in railway systems
Possible electromagnetic interferences in the new High-Speed
railway systems of the Italian railways generated by dc
conventional railway systems have been studied. The research
activity outlined the main interference electromagnetic sources and
the provisions to adopt in the design stage of the High-Speed lines
in order to limit possible electromagnetic couplings. Recently,
Leonardo Sandrolini participated to a PRIN (research Project of
Relevant National Interest) project and presented a contribution to
the Workshop WS1 "EMC Management and Assurance in Railway" at the
18th Int. Zurich Symp. on Electromagnetic Compatibility, Munich,
Germany, Sep. 24-28 2007. In this contribution the main EMI sources
in a railway system were classified. Moreover, a study on possibile
EMI situations between two different railway signalling systems was
carried out.
- Electromagnetic characterization of materials
(b) electrical properties of dispersive materials to predict their
shielding effectiveness to electromagnetic waves
The research activity in this field, in partial collaboration with
foreign industry researchers, concerns the electrical
properties of dispersive materials with the aim to predict their
shielding effectiveness to electromagnetic waves. Energy loss
inside the material is taken into account by means of a complex
permittivity, whose real part is an effective permittivity and
whose imaginary part is a term that depends on the effective
electrical conductivity of the material and is inversely
proportional to the frequency. Effective permittivity and effective
electrical conductivity can be measured and used to predict the
shielding effectiveness of a planar infinite shield by means of the
transmission line theory. Nonideal shields can still be studied
with numerical simulators and the electrical properties of the
materials can be represented by theoretical analytical models. In
particular, construction materials were studied, with reference to
concrete. Among the models implemented in numerical simulators, the
most appropriate one to represent the electric properties of
concrete is the extended Debye model, where an additional term is
introduced in the imaginary part of the complex permittivity to
take explicit account of the energy loss due to the material
electrical conductivity. The parameters (effective permittivity and
effective conductivity) of the Debye model were obtained by
fitting to experimental values with a nonlinear least-squares
Marquardt–Levenberg algorithm. The comparison between the shielding
effectiveness obtained with a commercial TLM numerical simulator
and that obtained analytically with the classical transmission-line
theory showed a good agreement for concrete with different moisture
content.
(b) electrical properties of adhesive materials for restorative
dentistry
The application of an electric field has been shown to positively
influence the impregnation of the resin monomers currently used in
dentin bonding systems during hybrid layer formation. This study
presents an experimental characterization of the electrical
properties of these monomers with the aim of both correlating them
to their chemical structures and seeking an insight into the
mechanisms of the monomer migration under an applied electric
field.
- Electrical aspects of energetics
(a) electrical characterization of renewable sources of electrical
energy
This research activity has been carried out mainly within two
research projects financed by both the Italian Ministry for the
University and the University of Bologna. Leonardo Sandrolini
started a research activity focused at the development of a
nonlinear equivalent circuit for a 1.2 kW polymer electrolyte
membrane (PEM) fuel cell stack available at the Department of
Electrical Engineering of the University of Bologna. The model
allows the simulation of both steady-state and dynamic behaviour of
the stack on condition that the values of some of its parameters
are changed in the two operating conditions. The circuit parameters
can be obtained by means of simple experimental tests and
calculations. The commercial PEM fuel cell stack is modelled as
seen from the power conditioning system side, without requiring
parameters necessary for complex mathematical models and not easily
obtainable by the majority of users. The equivalent circuit can be
used in power generation systems to verify their stability in
normal and fault operating conditions.
Furthermore, the research concerned the electrical characterization
of photovoltaic (PV) modules whose experimental current-voltage
characteristic is known. A distributed computing technique has been
implemented to extract the parameters of the equivalent circuit,
that can be useful during the design stage (to design and improve
the devices) as well as to integrate these energy sources in more
complex systems. A trial current-voltage characteristic, that
represent the output voltage of the PV module equivalent circuit,
is fitted to the experimental characteristic by means of an
iterative algorithm based on the particle swarm optimization. In
the algorithm, interlaced computing is exploited to decrease the
computational load, whereas parallel computing is used to collect
as many data as possible and save computational time. Statistics
applied on the obtained data allows one to control the typical
instability of numerical fitting algorithms.
(b) wireless transmission of electrical energy with a nonradiative
technique
The research concerns an accurate characterization of a wireless
power transfer system consisting of two resonant air-core coils
mutually coupled in free space. The lumped-circuit parameters of
the equivalent circuit are determined with analytical formulas
taken from the literature and validated by comparison with
numerical simulations with a finite-element computer code and with
experiments. The parameters are determined taking as input only the
geometry of the system (coil size and mutual distance, conductor
radius, and turn distance) and the frequency.
- Electromagnetic field analysis
The research concerns the implementation of a procedure which
exploits a sufficient condition for determining X-points in
quasistatic magnetic induction fields characterized by
translational or axisymmetrical geometry.
