Contribution à l'étude des modes de rayonnement acoustique d'une structure

Noise radiated by different industrial structures that surround us in daily life are more and more considered as environmental pollution. Standards defining a tolerable sound level for each of these noise sources are regularly called into question and respecting them becomes an additional constraint for manufacturers. The last decades have seen the increasing development of means used to fight these noise disturbances. The wide diversity of noises perceived as harmful has contributed to the progressive increase in specificity and efficiency of solutions proposed to reduce the disturbances. For some applications, passive solutions, based on the use of materials capable of absorbing or deviating acoustic or vibratory waves, have progressively been replaced by "active" solutions, based on the generation of an acoustic wave of opposite phase to the disturbing one radiated by the noise source. Power transformers are sources for which passive solutions (anti-noise walls) are usually expensive and not very efficient, particularly at low frequency. Furthermore, characteristics of noise radiated by transformers (low frequency tone noise) are such that they are particularly well suited for the implementation of an active solution. Typically, an active control system dedicated to transformers (feedforward) is composed of actuators, used to generate the anti-noise and usually located near the tank, sensors, used to measure the attenuation obtained and to provide a reference signal, and a controller used to drive the actuators as a function of the information collected by the sensors. When the sensors are microphones, they are usually moved away from the noise source, in such a way that they pick up acoustic pressure that is representative of the noise propagated far away. In the vicinity of the source, local acoustic phenomena can occur which are not propagated far away. A microphone located near the noise source would therefore pick up an acoustic pressure that did not necessarily represent the one that effectively exists far away, the area where we are in fact seeking to reduce the noise. These phenomena, frequently grouped under the term "nearfield" tend to decrease the performance of the control system, owing to the fact that the controller seeks in this case to reduce an acoustic pressure which is not representative of the noise to be reduced. In practice, the significant amount of wiring required as a result of the positioning of the microphones in the far field, has a non-negligible effect on the cost of the system. The possibility of bringing the sensors closer to the source is sufficiently advantageous to envisage to studying the feasibility and the resulting consequences. The present approach consists in representing the primary field radiated by a vibrating structure in terms of a set of "acoustic radiation modes". The use of radiation modes to characterise the behaviour of a structure has received increasing attention since the beginning of 90's, especially for active control applications. Usually, this approach consists in representing the radiated power in terms of a set of surface velocity distributions, called radiation modes, that have the property of radiating independently of each other. Radiation modes result from diagonalization of a discrete expression of the radiation operator. We propose here to study the consequences on the radiation modes of a structure from bringing the microphones closer to it. We will study how the acoustic field varies with the distance and how this can be used to obtain a model, the complexity of which is adapted to the observation distance.

The MOVE System: Monitoring of waste collecting vehicles

Telmo Cunha, Phillip Tomé

Nowadays, efficiency and productivity are fundamental concepts, in any kind of business. Although one can easily accept this as a known fact, one cannot easily identify which steps must be taken to achieve these goals. One approach consists of taking full advantage of the assets already available in the company, be they manpower or equipment. One kind of business that is strongly exposed to this problem concerns urban waste collecting. In this case, the productivity of the company can increase substantially if good fleet management policies are followed. Being aware of this fact, GEONAV has developed a fleet management system, named MOVE (Vehicle Monitoring), which is based on GPS. Each vehicle is equipped with a small onboard computer that includes, in the less complex model, a 12 channel GPS receiver and a GSM module. To cope with the problems usually affecting GPS in urban environments, such as having few satellites in view and multipath due to urban canyons, the system can incorporate information from low cost dead-reckoning sensors, such as the vehicle odometer and a gyroscope. Furthermore, other features can be added to the system, by using information from sensors readily available in the vehicle or that are installed specifically for the task intended. For instance, through a keypad unit, the driver can identity itself, by inserting a specific code or input the amount of fuel supplied to the vehicle. In addition, the system can automatically count the number of waste bins emptied, the number of times the vehicle has disposed of its waste in the wasteland or determine in real-time the vehicle’s weight. All information is sent to a Control Center using GSM or is stored in the computer’s memory, for a later download. In this case, an alternative means of transmission besides GSM can be used, namely a short range radio. Since no costs are involved when using this radio system, the exploitation costs of the MOVE system are greatly reduced. The Control Center software is based on a client/server architecture. The server application is responsible for controlling the communications between the Control Center and the vehicles and for storing all data received in a central database. The client applications connect to the server application, using the intranet, and are based on the GIS ArcView 8.x platform. Using a client application, an operator can visually determine the fleet status over a map or other type of visual representation. To reduce exploitation costs of the system, the status of the fleet is obtained by using SMS messages to exchange data between the vehicles and the Control Center. However, the option to follow a particular vehicle, by establishing a GSM call, is also possible. In this case, the position of the vehicle is updated at 1 Hz. The information that is stored in the database is organized in such a way as to allow the calculus of several statistical parameters that are useful to the management and control of the fleet. For example, one can determine the amount of kilometers driven, the time to complete a particular waste collection circuit, the fuel consumption, the number of waste bins emptied and their location, the number of times the vehicle was emptied of its waste, the vehicle’s weight, etc. This information can be used to establish new and better policies to enhance the fleet’s productivity and to present a better service to the end customer. This paper describes in detail the architecture of the MOVE system and its functionalities, including both the equipment installed onboard the vehicles and the Control Center’s software. Emphasis is given to the system’s flexibility to be used in different case scenarios, besides the monitoring of waste collection vehicles.

2003

Mobility-centric design of collaborative transportation systems

Embedded sensors and actuators are revolutionizing the way we perceive and interact with the physical world. Current research on such Cyber-Physical Systems (CPSs) has focused mostly on distributed sensing and data gathering. The next step is to move from passive information extraction from the physical world towards an active framework where information is retrieved, processed, and acted upon in situ. As an example of such sensor-actuator systems that provide large-scale, distributed coordination, we consider Intelligent Transportation Systems (ITSs), with the goal of increasing travel safety and efficiency. The proliferation of wireless technologies enables different actors (e.g., pedestrians, motorists, traffic operators) to communicate with each other cheaply, efficiently, and securely. By embedding computational intelligence, communication and control into such ITSs, it will be possible to build collaborative transportation applications that help solve, for example, congestion and parking problems. This thesis explores the following question: To what extent is it possible to build distributed ITS systems that rely exclusively on local communication between nodes? The potential benefits of building such systems without infrastructure are numerous, including lower fixed and variable costs, avoiding regulatory hurdles, lower entry barriers for new players, and more control over privacy. On the other hand, self-organized ITSs without infrastructure have to operate under challenging conditions: large scale, high mobility, lack of end-to-end connectivity, ephemeral contacts between wireless nodes, and heterogeneous node capabilities. Although the networking research community has invested a significant effort in designing service abstractions that mimic traditional IP connectivity on top of wireless ad hoc networks, the ITS scenarios considered in this work are too challenging to implement such a service model. The main goal of this thesis is to design communication service models and their underlying protocols that can operate in collaborative transportation applications, and to demonstrate their effectiveness through analysis of traffic data and through realistic simulations. A key point is that the ITS applications we consider can rely on more limited service primitives, because they do not require a general any-to-any delivery service with strict performance guarantees. We first define the collaborative transportation applications of interest. Then we quantify their potential benefits and discuss their functional requirements. Next, we focus on the problem of the collection of large-scale mobility data. We propose a new method for collecting such data and introduce a novel mobility data mining framework, which is necessary to study collective mobility patterns in the context of wireless ad hoc networking. Relying on the analysis of real-life mobility traces, we find that despite the high node mobility, clusters of time-stable connectivity emerge and last at specific locations. Outside such clusters the connectivity between nodes remains sparse. This leads us to the proposal of a new mobility model that captures in an elegant way two phenomena observed in reality, i.e., emergence of stable clusters and network partitioning. This mobility model, called Heterogeneous Random Walk (HRW), appears to be the worst-case mobility model for many mobility-assisted protocols. Moreover, we show that the HRW mobility model predicts the performance of an epidemic dissemination protocol more accurately than other, similarly parsimonious models. Based on the lessons learned from the mobility data analysis, we propose a new abstraction that allows us to capture collective mobility patterns. This abstraction, called a Mobility Map, can be shared globally among mobile nodes and it can be used to improve communication in mobile partitioned networks (MPNs). Notably, we present a new geocasting protocol called GeoMobCast, which uses Mobility Maps to minimize message delay. This protocol is designed to provide the users of the collective transportation applications with the required communication service. Finally, we turn our attention on the real-life implementation of the distributed ITS that leverage short-range wireless communication. To this extent, we present the results of our experimental work with wireless sensor network technologies. We also present the necessary simulation toolbox designed to implement and evaluate collaborative transportation applications.

EPFL2009

Signal Processing in Space and Time

Francisco Pereira Correia Pinto

Sound waves propagate through space and time by transference of energy between the particles in the medium, which vibrate according to the oscillation patterns of the waves. These vibrations can be captured by a microphone and translated into a digital signal, representing the amplitude of the sound pressure as a function of time. The signal obtained by the microphone characterizes the time-domain behavior of the acoustic wave field, but has no information related to the spatial domain. The spatial information can be obtained by measuring the vibrations with an array of microphones distributed at multiple locations in space. This allows the amplitude of the sound pressure to be represented not only as a function of time but also as a function of space. The use of microphone arrays creates a new class of signals that is somewhat unfamiliar to Fourier analysis. Current paradigms try to circumvent the problem by treating the microphone signals as multiple "cooperating" signals, and applying the Fourier analysis to each signal individually. Conceptually, however, this is not faithful to the mathematics of the wave equation, which expresses the acoustic wave field as a single function of space and time, and not as multiple functions of time. The goal of this thesis is to provide a formulation of Fourier theory that treats the wave field as a single function of space and time, and allows it to be processed as a multidimensional signal using the theory of digital signal processing (DSP). We base this on a physical principle known as the Huygens principle, which essentially says that the wave field can be sampled at the surface of a given region in space and subsequently reconstructed in the same region, using only the samples obtained at the surface. To translate this into DSP language, we show that the Huygens principle can be expressed as a linear system that is both space- and time-invariant, and can be formulated as a convolution operation. If the input signal is transformed into the spatio-temporal Fourier domain, the system can also be analyzed according to its frequency response. In the first half of the thesis, we derive theoretical results that express the 4-D Fourier transform of the wave field as a function of the parameters of the scene, such as the number of sources and their locations, the source signals, and the geometry of the microphone array. We also show that the wave field can be effectively analyzed on a small scale using what we call the space/time-frequency representation space, consisting of a Gabor representation across the spatio-temporal manifold defined by the microphone array. These results are obtained by treating the signals as continuous functions of space and time. The second half of the thesis is dedicated to processing the wave field in discrete space and time, using Nyquist sampling theory and multidimensional filter banks theory. In particular, we show examples of orthogonal filter banks that effectively represent the wave field in terms of its elementary components while satisfying the requirements of critical sampling and perfect reconstruction of the input. We discuss the architecture of such filter banks, and demonstrate their applicability in the context of real applications, such as spatial filtering and wave field coding.

EPFL2010