Forests as natural metamaterial barriers for urban railway-induced vibration attenuation.

Noise and vibrations generated by railway traffic can seriously affect the adjacent buildings and their residents. Different mitigation methods have been proposed in the past decades to tackle this challenge. Despite many mitigation measures presented in the literature, some of these measures have shown limitations in their application, while for others their carbon footprint does not justify their implementation in real projects. This study introduces the concept of forests as natural metamaterials to attenuate the vibrations generated at the wheel-rail interaction. In particular, a group of natural metamaterials, in the form of a forest, is introduced into a vehicle/track/soil validated model based on the two-step approach. The ideal tree/soil unit-cell constituting the forest is obtained through a parametric investigation of the geometrical and material properties in order to have the first band-gap within the main range of frequencies generated by railway traffic in urban areas. The vibration attenuation levels obtained by the introduction of the natural metamaterial are then evaluated by considering a range of operational velocities for the T2000 Brussels tram LRV (Light Rail Vehicle). Finally, some insights on the attenuation efficiency of the selected forest towards vibrations generated by HSTs (High-Speed Trains) are given by considering a mono-wheel model with a higher range of vehicle speeds.

Improved analysis of ground vibrations produced by man-made sources.

Man-made sources of ground vibration must be carefully monitored in urban areas in order to ensure that structural damage and discomfort to residents is prevented or minimised. The research presented in this paper provides a comparative evaluation of various methods used to analyse a series of tri-axial ground vibration measurements generated by rail, road, and explosive blasting. The first part of the study is focused on comparing various techniques to estimate the dominant frequency, including time-frequency analysis. The comparative evaluation of the various methods to estimate the dominant frequency revealed that, depending on the method used, there can be significant variation in the estimates obtained. A new and improved analysis approach using the continuous wavelet transform was also presented, using the time-frequency distribution to estimate the localised dominant frequency and peak particle velocity. The technique can be used to accurately identify the level and frequency content of a ground vibration signal as it varies with time, and identify the number of times the threshold limits of damage are exceeded.

Review of Trackside Monitoring Solutions: From Strain Gages to Optical Fibre Sensors.

A review of recent research on structural monitoring in railway industry is proposed in this paper, with a special focus on stress-based solutions. After a brief analysis of the mechanical behaviour of ballasted railway tracks, an overview of the most common monitoring techniques is presented. A special attention is paid on strain gages and accelerometers for which the accurate mounting position on the track is requisite. These types of solution are then compared to another modern approach based on the use of optical fibres. Besides, an in-depth discussion is made on the evolution of numerical models that investigate the interaction between railway vehicles and tracks. These models are used to validate experimental devices and to predict the best location(s) of the sensors. It is hoped that this review article will stimulate further research activities in this continuously expanding field.

Real-time measurement of dynamic accelerations with optical-fiber accelerometer based on polarization analysis.

This paper presents a novel technique for the measurement of accelerations using an optical-fiber accelerometer based on the analysis of polarization variations. The technique relies on the identification of six parameters of the fiber Mueller matrix through polarimetric measurements. We demonstrate by theoretical and experimental work that the sensitivity of the proposed method does not depend on the input polarization states sent into the fiber nor on the intrinsic fiber birefringence. The sensor has been successfully tested for acceleration amplitudes between 2.5 and 20 m/s² at a frequency of 120 Hz. The acceleration resolution increases from 0.1 m/s² around an acceleration of 2.5 m/s² to 0.35 m/s² around 20 m/s². The acceleration measurements can be performed every 160 µs.

Development of a multi-point polarization-based vibration sensor.

In this paper we propose a novel kind of multi-point vibration sensor based on the polarization properties of light. Its principle relies on the combination of mechanical transducers with fiber Bragg gratings. When subject to vibrations, the mechanical transducers induce birefringence variations within the fiber and in turn modify the state of polarization, which appears as a power variation after going through a polarizer. The FBGs reflect light from different positions of the sensing fiber and provide wavelength multiplexing. We show that this sensor can provide the vibration frequencies in a quasi-distributed manner.

Linearity considerations in polarization-based vibration sensors.

In this paper, the characteristics of a polarization-based vibration sensor are theoretically and experimentally analyzed with a focus on its sensitivity and linearity. It is shown that this sensor can correctly recover the vibration frequency spectrum (i.e., with limited distortions) up to an acceleration of 140 m/s(2), with a sensitivity equal to 9.98 mV/(m/s(2)).

Temperature-insensitive polarimetric vibration sensor based on HiBi microstructured optical fiber.

A new type of highly birefringent microstructured optical fiber has been tested for vibration measurements using a polarimetric technique. This technique takes advantage of the stress-induced phase shift between the two orthogonally polarized fiber eigenmodes. Comparison of three different fiber types shows that standard single-mode fibers do not provide stable measurements and that conventional polarization-maintaining fibers lead to a significant cross-sensitivity to temperature. However, for highly birefringent microstructured fibers specifically designed to provide a temperature-independent birefringence, our experiments show repeatable vibration measurements over a frequency range extending from 50 Hz to 1 kHz that are unaffected by temperature variations (up to 120 °C).

An active foot lifter orthosis based on a PCPG algorithm.

Central pattern generators (CPGs) are known to play an important role in the generation of rhythmic movements in gait, both in animals and humans. The comprehension of their underlying mechanism has led to the development of an important family of algorithms at the basis of autonomous walking robots. Recently, it has been shown that human gait could be modeled using a subclass of those algorithms, namely a Programmable Central Pattern Generator (PCPG). In this paper, we present a foot lifter orthosis driven by this algorithm. After a learning phase, the PCPG is able to generate adequate rhythmic gait patterns both for constant speeds and acceleration phases. Its output is used to drive the orthosis actuator during the swing phase, in order to help patients suffering from foot drop (the orthosis just follows the movement during the stance phase). The most interesting property of this algorithm is the possibility to generate a smooth output signal even during speed transitions. In practice, given that human gait is not perfectly periodic, the phase of this signal needs to be reset with actual movement. Therefore, two phase-resetting procedures were studied: one standard hard phase-resetting leading to discontinuities and one original soft phase-resetting allowing to recover the correct phase in a smooth way. The simulation results and complete design of the orthosis hardware and software are presented.