The development of precise and robust navigation strategies for Autonomous Underwater Vehicles (AUVs) is fundamental to reach the high level of performance required by complex underwater tasks, often including more than one AUV. One of the main factors affecting the accuracy of AUVs navigation systems is the algorithm used to estimate the vehicle motion, usually based on kinematic vehicle models and linear estimators. A precise and reliable navigation system is indeed fundamental to AUVs: the Global Positioning System (GPS) signal is not available underwater, thus making it very hard to know the position of the vehicle in real-time. In this paper, the authors present an innovative navigation strategy specifically designed for AUVs, based on the Unscented Kalman Filter (UKF). The new algorithm proves to be effective if applied to this class of vehicles and allows us to achieve a satisfying accuracy improvement compared to standard navigation algorithms. The proposed strategy has been experimentally validated using the navigation data acquired in suitable sea tests performed in Biograd Na Moru (Croatia) in the framework of the FP7 European ARROWS project tests performed during the Breaking the Surface 2014 (BtS 2014) workshop. The vehicles involved are the two Typhoon AUVs, developed and built by the Department of Industrial Engineering of the University of Florence within the THESAURUS Tuscany Region project for exploration and surveillance of underwater archaeological sites. The experiment, described in the paper, was performed to preliminary test the cooperative navigation between these AUVs. The new algorithm has been initially tested offline, and the validation of the proposed strategy provided accurate results in estimating the vehicle dynamic behaviour
a b s t r a c tThe prediction of the wear at the wheel-rail interface is a fundamental problem in the railway field, mainly correlated to the planning of maintenance interventions, vehicle stability and the possibility of researching specific strategies for the wheel and rail profile optimization. In this work the Authors present a model specifically developed for the evaluation of the wheel and rail profile evolution due to wear, whose layout is made up of two mutually interactive but separate units: a vehicle model for the dynamic analysis and a model for the wear estimation. The first one is made up of two parts that interact online during the dynamic simulations: a 3D multibody model of the railway vehicle implemented in Simpack Rail (a commercial software for the analysis of multibody systems) and an innovative 3D global contact model (developed by the Authors in previous works) for the detection of the contact points between wheel and rail and for the calculation of the forces in the contact patches (implemented in C/C++environment). The wear model, implemented in the Matlab environment, is mainly based on experimental relationships found in literature between the removed material and the energy dissipated by friction at the contact. It starts from the outputs of the dynamic simulations (position of contact points, contact forces and global creepages) and calculates the pressures inside the contact patches through a local contact model (FASTSIM algorithm); then the material removed due to wear is evaluated and the worn profiles of wheel and rail are obtained. This approach allows the evaluation of both the quantity of removed material and its distribution along the wheel and rail profiles in order to analyze the development of the profiles shape during their lifetime.The whole model is based on a discrete process: each discrete step consists in one dynamic simulation and one profile update by means of the wear model while, within the discrete step, the profiles are supposed to be constant. The choice of an appropriate step is fundamental in terms of precision and computational load. Moreover the different time scales characterizing the wheel and rail wear evolution require the development of a suitable strategy for the profile update: the strategy proposed by the Authors is based both on the total distance traveled by the considered vehicle and on the total tonnage burden on the track. The entire model has been developed and validated in collaboration with Trenitalia S.p.A. and Rete Ferroviaria Italiana (RFI), which have provided the technical documentation and the experimental results relating to some tests performed with the vehicle DMU Aln 501 Minuetto on the Aosta-Pre Saint Didier line.
The multibody simulation of railway vehicle dynamics needs a reliable and efficient method to evaluate the contact points between wheel and rail, because their positions have a considerable influence on the direction and intensity of the contact forces. In this work, an innovative semi-analytic procedure for the detection of the wheel/rail contact points (named the DIFF method) is presented. This method considers the wheel and the rail as two surfaces whose analytic expressions are known and is based on the idea that in the contact points the difference between the surfaces has local minima and is equivalent to solving an algebraic two-dimensional system. The original problem can be reduced analytically to a simple scalar equation that can be easily solved numerically (since the problem dimension is one, even elementary non-iterative algorithms can be efficient).
a b s t r a c tThe multibody simulation of railway dynamics needs a reliable and efficient method to properly describe the contact between wheel and rail.In this work innovative methods to evaluate the position of contact points are presented. The aim is to develop a method which can be implemented on-line, assuring a calculation time consistent with real-time calculations of multibody dynamics. At the same time it has to be very accurate, to properly predict the local forces at contact in order to describe even the wear of contact surfaces.In this work the authors present two different approaches to find stationary points during a multibody simulation. In the former the conditions to define a local minima are wrote in an analytical way. This makes possible to combine the conditions in order to reduce the analytic problem's dimension and then to solve numerically the problem with a low computational burden. The latter approach calculates the location of local minima using a method based on neural networks. The paper will cover the details of the proposed methods and the performances, in terms of computation time and accuracy, will be compared with those of the conventional algorithms used by commercial softwares, showing their reliability and low computational burden. Moreover, an implementation of the proposed models in a multibody simulator will be presented, in order to show their suitability for this application.
The multibody simulation of railway vehicle dynamics needs a reliable and efficient method to determine the location of the contact points between wheel and rail that represent the application points of the contact forces and influence their directions and intensities. In this work, two semi-analytic procedures for the detection of the wheel-rail contact points (named the DIST and the DIFF methods) are presented. Both the methods consider the wheel and the rail as two surfaces whose analytic expressions are known. The first method is based on the idea that the contact points are located in the point in which the distance between the contact surfaces has local maxima, and is equivalent to solve an algebraic 4D-system. The second method is based on the idea that in the contact points the difference between the surfaces has local minima and is equivalent to solve an algebraic 2D-system. In both cases, the original problem can be reduced analytically to a simple 1D-problem that can be easily solved numerically.
The numerical wheel wear prediction in railway applications is of great importance for different aspects, such as the safety against vehicle instability and derailment, the planning of wheelset maintenance interventions and the design of an optimal wheel profile from the wear point of view. For these reasons, this paper presents a complete model aimed at the evaluation of the wheel wear and the wheel profile evolution by means of dynamic simulations, organised in two parts which interact with each other mutually: a vehicle's dynamic model and a model for the wear estimation. The first is a 3D multibody model of a railway vehicle implemented in SIMPACK™, a commercial software for the analysis of mechanical systems, where the wheel–rail interaction is entrusted to a C/C++user routine external to SIMPACK, in which the global contact model is implemented. In this regard, the research on the contact points between the wheel and the rail is based on an innovative algorithm developed by the authors in previous works, while normal and tangential forces in the contact patches are calculated according to Hertz's theory and Kalker's global theory, respectively. Due to the numerical efficiency of the global contact model, the multibody vehicle and the contact model interact directly online during the dynamic simulations.\ud \ud The second is the wear model, written in the MATLAB® environment, mainly based on an experimental relationship between the frictional power developed at the wheel–rail interface and the amount of material removed by wear. Starting from a few outputs of the multibody simulations (position of contact points, contact forces and rigid creepages), it evaluates the local variables, such as the contact pressures and local creepages, using a local contact model (Kalker's FASTSIM algorithm). These data are then passed to another subsystem which evaluates, by means of the considered experimental relationship, both the material to be removed and its distribution along the wheel profile, obtaining the correspondent worn wheel geometry.\ud \ud The wheel wear evolution is reproduced by dividing the overall chosen mileage to be simulated in discrete spatial steps: at each step, the dynamic simulations are performed by means of the 3D multibody model keeping the wheel profile constant, while the wheel geometry is updated through the wear model only at the end of the discrete step. Thus, the two parts of the whole model work alternately until the completion of the whole established mileage. Clearly, the choice of an appropriate step length is one of the most important aspects of the procedure and it directly affects the result accuracy and the required computational time to complete the analysis.\ud \ud The whole model has been validated using experimental data relative to tests performed with the ALn 501 ‘Minuetto’ vehicle in service on the Aosta–Pre Saint Didier track; this work has been carried out thanks to a collaboration with Trenitalia S.p.A and Rete Ferroviaria Italiana, which have provided the necessary techn...
a b s t r a c tA detailed description of adhesion is crucial in tribology, vehicle dynamics and railway systems, both theoretically and practically. However, an accurate adhesion model is quite hard to develop because of the complex and non-linear behaviour of the adhesion coefficient and the external unknown contaminants which are present between the contact surfaces. The problem becomes even more complicated when degraded adhesion and large sliding between the contact bodies (for instance wheel and rail) occur.In this paper the authors describe an innovative adhesion model aimed at increasing the accuracy in reproducing degraded adhesion conditions in vehicle dynamics and railway systems; the new approach turns out to be quite suitable also for multibody applications (fundamental in this research topic). The model studied in the work considers some of the main phenomena behind the degraded adhesion: the large sliding at the contact interface, the high energy dissipation, the consequent cleaning effect on the contact surfaces and, finally, the adhesion recovery due to the external unknown contaminant removal.The new adhesion model has been validated through experimental data provided by Trenitalia S.p.A. and coming from on-track tests carried out in Velim (Czech Republic) on a straight railway track characterised by degraded adhesion conditions. The tests have been performed with the railway vehicle UIC-Z1 equipped with a fully-working Wheel Slide Protection (WSP) system.The validation showed the good performances of the adhesion model both in terms of accuracy and in terms of numerical efficiency; high computational performances are required to implement the developed model directly online within more general and complex multibody models (e.g. in MatlabSimulink and Simpack environments). In conclusion, the adhesion model highlighted the capability of well reproducing the complex phenomena behind the degraded adhesion.
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