Many applications are characterized by very severe environment from the sensors point of view. An example is the railway field, where often the measurement setup is subjected to high voltage and electromagnetic emission. In these work conditions, optical sensors are more suitable than electrical ones due to their characteristics. In this paper, the acceleration measurements are taken into account. Nowadays many optical accelerometers are present on the market. In particular, in this work FBG accelerometers are considered. This kind of sensors is characterized by an undamped resonance that can produce the break of the sensor. One possibility to avoid this problem is to add a damping effect in order to reduce the resonance amplification. In the following section, the method to reproduce a mechanical filter able to increase the damping coefficient of the sensor is presented. Experimental results in laboratory will be discussed, showing the effectiveness of the solution.
Giant Magnetostrictive Actuators (GMA) can be profitably used in application of vibration control on smart structures. In
this field, the use of inertial actuators based on magnetostrictive materials has been consolidate. Such devices turn out to
be very effective in applications of vibration control, since they can be easily paired with sensors able to ensure the
feedback signal necessary to perform the control action.
Unlike most widespread applications, this paper studies the use of patch magnetostrictive actuators. They are made of a
sheet of magnetostrictive material, rigidly constrained to the structure, and wrapped in a solenoid whose purpose is to
change the intensity of the magnetic field within the material itself.
The challenge in the use of such devices resides in the impossibility of having co-located sensors. This limit may be
exceeded by using strain gauge sensors to measure the deformation of the structure at the actuator. This work analyzes
experimentally the opportunity of introducing, inside a composite material structure, both the conventional electric strain
gauges and the less conventional optical sensors based on Bragg’s gratings.
The performance of both solutions are analyzed with particular reference to the signal to noise ratio, the resolution of the
sensors, the sensitivity to variations of the electric and magnetic fields and the temperature change associated with the
operation of the actuator.
Embedding FBG sensors in carbon fiber structures is a very attractive solution, due to the small fiber diameter, and the possibility to manufacture arrays of many gratings into a single optical fiber. These embedding is particularly useful for the manufacturing of smart structures, able to improve their characteristics thanks to embedded sensors and actuators. In this work a carbon fiber beam of 3 m length, with an array of 30 FBG sensors and 3 piezoelectric actuators, is described. The focus of the work is on the evaluation of the good operation of embedded FBG sensors, that is not easy due to the microstructure of woven carbon fiber layers, producing non-homogeneous strain field, a well known problem for the reliability of FBG strain measurements. The proposed technique looks at the standard deviation of the full width at -6 dB of the spectra of each FBG sensors, during a quasi-static motion producing quasi-static strains. 37% of the 30 FBG sensors have been found to produce measurements corrupted by a small error. At the end, vibration control of the described structure is shown.
The contact force between pantograph and catenary is a parameter needed to evaluate the quality of the dynamic interaction of the two systems. The in-line measurements of this parameter have always been very hard due to the particular work condition of pantograph and catenary. High voltage (from 1.5 kV dc for underground trains to 25 kV ac for high-speed trains) is the main obstacle to be overcome for the measurement setup: if traditional sensors are used (electrical load cells), an adequate electrical insulation of the whole measurement setup is necessary, making it more complex and more expensive. In this work, the use of fiber Bragg gratings (FBG) load cells, designed on purpose, is presented. The main advantage of this kind of sensor is the intrinsic electrical insulation, which makes it very suitable for the measurement of pantograph-catenary contact force. In the following sections, laboratory tests and in-line tests using the optical load cells to measure the pantograph-catenary contact force will be discussed, proving the good work of the load cells and the possibility to obtain an easier measurement setup for the considered application.
Carbon-fiber structures has many advantages over traditional materials, but special care must be taken when bonded
joints are used. A crack can propagate and cause the failure of the structure. The presence of a crack and its length can be
monitored in example by means of backface strain measurements. In this work the backface technique is applied by
using an array of 10 Fiber Bragg Grating (FBG) sensors applied on a carbon-fiber bonded joint. This is an interesting
solution because these sensors can be embedded in carbon-fiber structures nearly without producing a load effect thanks
to their small cross section. Special care must be used with FBG sensors because the expected strain profile is
non-homogeneous (computed using a FEM analysis) and the FBG spectra can be altered and prevented from correct
measurements (simulations using a T-matrix model were conducted). The specimen was tested with a pulsating tensile
force, so that a crack developed and propagated below the optical sensors. The comparison of the observed crack
position and the one inferred from the measured position of the minimum of the longitudinal strain pattern showed a
good correlation, with errors lower than 10% except a small range where a 20% error was observed.
KEYWORDS: Fiber Bragg gratings, Sensors, Actuators, Control systems, Laser applications, Time metrology, Vibration control, Signal processing, Data acquisition, Carbon
FBG sensors are an attractive solution for vibration control applications since they can provide a large number of
measurements with a negligible effect on the mechanical properties of the controlled structure and without problems
related to measurement layout and overall dimensions. The paper analyzes the limits on the use of swept laser FBG
interrogators for control systems applications due to time delays. Results show that good control performance can be
obtained at least up to 20 Hz.
The measurement of the contact force between pantograph and catenary in railway engineering has been carried out in
the past with conventional electrical load cells. This study evaluates the use of fiber Bragg gratings (FBG) sensors, so
that electrical insulation of the load cells, placed on high voltage parts, is obtained by means of the fiber optic intrinsic
insulation. While FGB sensors can be compared to traditional resistance strain gauges, their dimensional and strain
profile characteristics required the development of a novel load cell design. A trade-off between two opposite
requirements were made: an adequate sensitivity of the FBG output (high strains) and a high reliability (small strains).
Moreover the application requested predefined dimensional constrains. The selected design is basically an S-shaped load
cell. Static and dynamic calibrations were performed also to asses the separation of the effects due to axial force, bending
moment and temperature. Both test rig and in-service measurements has been performed, and the very first results show
the good operation. The main metrological characteristics are: design full scale of ±500 N with a static safety factor of 5,
equivalent spectral noise of 0.064 N/√Hz, first resonance frequency of mounted cell of about 200 Hz (so that a useful
range of at least (0 to 50) Hz is obtained).
Fiber optic strain sensors, such as Fiber Bragg Gratings (FBG), have a great potential in the use in smart
structures thanks to their small transversal size and the possibility to make an array of many sensors. They can
be embedded in carbon fiber structures and their effect on the structure is nearly negligible. On the other hand,
some critical aspects should be evaluated, such as higher cost and lower SNR respect to traditional technologies.
In this work a simple carbon fiber structure has been developed, composed of a thin cantilever with 14 longitudinal
FBG sensors and 3 piezoelectric actuators (PZT). A dynamic 1 kHz swept-laser interrogator was used to gather
the FBG data: the output is a digital signal and the time delay introduced has been measured. This is a critical
point that restricts the highest controllable frequency: our results show that the limit is at about 50 Hz. A
control system has been developed and many control strategies have been evaluated to suppress vibration, from
the simplest single-sensor to single-actuator strategy, up to modal control. The SNR of the input data has been
found to be critical. The use of FBG sensors allows improving the performance of the control because they give
a large number of measurements regarding the state of deformation of the whole structure. Modal control was
found to have the best results thanks to its best use of all the sensors data.
Carbon fiber structures are claimed to offer several advantages such as contained mass, high stiffness and low thermal
expansion. However, these structures are characterized by a very low mechanical damping and, therefore, they are easily
subjected to potentially dangerous vibratory phenomenon. Active control techniques have been widely developed to
suppress vibration and great progresses have been achieved. On the other hand the research on sensors and actuators to
be used is still a field of interest. The paper discusses the opportunity to use piezoelectric actuators (PZT) and Fiber
Bragg Grating sensors (FBG) to realize a smart structure including in itself both the sensing and the actuating devices.
Fiber optic strain sensors, such as Fiber Bragg Gratings, have a great potential in the use in smart structures thanks to
their small transversal size and the possibility to make an array of many sensors. Even if this is not the case of the
reported study, they can be embedded between carbon fiber layers and their effect on the structure is usually negligible.
Such a structure is able to measure its state of excitation and to reduce the amplitude of vibration using the PZT
actuators. Different control strategies have been implemented on a test rig consisting on a carbon fiber cantilever beam
with 14 FBG sensors and 3 PZT actuators. Control forces are designed to increase the damping of the structures,
allowing to increase of damping of the first modes of vibration of about 10 times.
The increasing use of Fiber Bragg Grating (FBG) in many fields, such as smart structures and structural health
monitoring, requires a detailed analysis of the measured values from the optical interrogators. While the peak detection
of each FBG reflection spectra is relatively simple if the strain profile is constant all over the FBG sensor, special care
must be paid when the strain profile is unknown, or non-homogeneous for sure. The objective of this work is to develop
a numerical model able to simulate the whole measurement process, from the strain profile to the measured peak
wavelength. The numerical simulation include a T-matrix model and some of the most used peak detection algorithms.
The results obtained with some of the tested strain profiles show that the measured errors can be important.
The incoming restoration works of Duomo di Milano main spire requires a continuous structural health monitoring of the
cupola supporting it. For reasons mainly connected to the lightning hazard, fiber optic sensors have been selected, based
on FBG technology. Strain of the lower part of the vaulting-rigs inside the octagonal cupola is the measurement of
interest. Being the expected signals very small and the thermal disturbances very important, a thermal characterization of
two types of commercial strain gauges was carried out in laboratory with a thermal chamber and a block of the same
marble used for the Duomo construction. This allowed to find a relationship later used to compensate any thermal
effects, leading to the extraction of the mechanical load contribution only. An uncertainty analysis gave a result of 5 to
10 μm/m in the tested temperature range -5 °C to +40 °C. The future work will expand the monitoring system to more
measurement points and it is expected this can provide an important diagnostic tool during restoration operations.
In railway engineering the monitoring of contact force between pantograph and catenary gives information about the
interaction between the two systems and it is useful to check the status of the overhead line. Indeed the failure of the
catenary is one of the main causes of out of order problems. This study was conducted in a test campaign on an
underground train instrumented with sensors able to monitor the line status. One of the more important measured
quantities is the pantograph contact force, and two measurement systems were implemented: one optical and another
electrical. The optical one was based on FBG sensors applied on the pantograph collector strip; the electrical one was
based on two load cells positioned at the sides of the collector strip. The in-line measurements show that the optical
solution is very promising, providing very reliable results that can be successfully used in the monitoring application,
allowing the determination of the critical point in the line. The thermal compensation of any FBG sensors is a known
problem and here is no exception: a thermal compensator was used to get also mean value measurements and the results
are discussed.
With the increase of the speed in the railway operation, a particular care has to be addressed to the interaction between
pantograph and catenary: the variation of contact force and, consequently, the contact loss probability increases. The
contact between pantograph and catenary is an electromechanical contact and when a contact loss occurs electrical arcing
phenomena arise. One of the most important effects of arcing phenomenon is the increase of wear for both contact wire
and contact strip. The monitoring of contact force between pantograph and catenary in the high frequency range (up to
values greater than 100 Hz) should be useful to put in evidence problems on the pantograph-catenary interface. In this
work the use of fiber optic strain gauges, with FBG technology, to measure the pantograph-catenary contact force is
discussed. This solution is insensitive to the electrical disturbances caused by the sparks and can be safely used in-line.
Starting from a static and dynamic validation of the sensors, using as a reference traditional electrical strain gauges, a
method to estimate the contact force was developed and tested.
The use of FBG based sensors for the monitoring of the pantograph-catenary interaction is very attractive due to the
insensitivity of fiber optic sensors to the electromagnetic disturbances and due to their ability to be electrically insulated.
In fact, the monitoring of pantograph-catenary interaction with traditional sensors needs a complicated set-up to
electrically insulate sensors, to power the signal conditioning devices and to transmit the signal to the data acquisition
system, and to avoid interferences between the measurement signals and electromagnetic disturbances typically
generated by continuous sparking and eventual arcing phenomena caused by contact loss between the pantograph
collector and the contact wire of overhead line. In this work the application of a commercial FBG accelerometer on a
pantograph of an underground train, instrumented for experimental in-line tests, is analyzed. In particular, a comparison
between a traditional capacitive accelerometer and a FBG accelerometer is presented to highlight the proper working of
the fiber optic sensor during in-line tests and to take the use of this kind of fiber optic sensor into consideration for
monitoring aim in the pantograph-catenary interaction, simplifying the measurement set-up. The first results show that
this approach is promising.
FBG sensors are particularly suitable for strain measurements on mechanical parts subjected to strong electromagnetic
disturbances. This is the case of strain measurements on the pantograph collector strip of underground trains, where, due
to arcing phenomena caused by contact loss between the pantograph collector and the contact wire of overhead line
during the current collection, high electromagnetic disturbances are present. Moreover, an intrinsic advantage of fiber
optic sensors is the ability to electrically insulate the sensor from the conditioning and data-gathering unit. In this work
the application of FBG sensors on the pantograph collector of an underground train, instrumented for experimental inline
tests, is presented. First, a FEM analysis of the collector to identify the suitable position, in terms of maximum
sensitivity and fiber safety, for the strain sensor was performed. The position of a thermal compensating FBG sensor was
also selected as a compromise between a place not too far from the main sensor (so that both are subject to the same
temperature), and with a small (or negative) strain. The compensated signal is simply computed subtracting the
compensator signal from the main one, and this can be considered roughly proportional to the total contact force. A static
calibration was conducted in the laboratory with standard masses, and dynamic tests were carried out to dynamically
characterize the pantograph. The FBG static measurements were compared with force measurements obtained from
traditional sensors positioned on the pantograph collector interface, on each side of the collector, in order to obtain
information on the position of vertical contact force. The total vertical contact force from the traditional load cells is
compared with the FBG measurement obtained in the center of the collector. The comparison of the total force signals
obtained during experimental results for both laboratory and in-line tests showed a good accordance, and these tests can
be considered as a validation of the method, so that it can be further developed and used in the future in similar
situations.
The Mars Infrared MApper (MIMA) is a FT-IR miniaturized spectrometer which is being developed for ESA ExoMars
Pasteur mission. MIMA will be mounted on the rover mast and so it must be compact and light-weight. The scientific
goals and its optical design are presented in two companion papers [1] [2]; the focus of this work is on the thermomechanical
design and testing. The instrument design faces challenging constraints both from the expected environment
and the allocated resources. The temperatures during operation are expected to be from -120 °C to +30 °C with the
presence of a low density but thermally effective atmosphere. Severe dynamic loads are foreseen during launch and
moreover at landing on Mars. The overall size is limited to an envelope of 140 mm x 140 mm x 120 mm and the mass to
less than 1 kg. The expected performances of this instrument should be comparable with those of much heavier ones
built in the past. An instrument compliant with these constraints has been conceived, introducing many innovative
solution with respect to the past experiences and making use of intensive modeling and testing to prove the survival to
the harsh environment.
Among the most challenging problems the mounting of the brittle KBr optics and the matching of its thermal expansion
coefficient with that of the supporting aluminium structure, in a temperature interval of more than 200 °C. Most of the
components have undergone thermovacuum tests in the low temperature range because none of them was expected to be
used in the -100 °C range.
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