http://hdl.handle.net/10993/81 Search this channel search http://orbilu.uni.lu/simple-search http://hdl.handle.net/10993/41905 Title: A Real-Time Approach for Chance-Constrained Motion Planning with Dynamic Obstacles <br/> <br/>Author, co-author: Castillo Lopez, Manuel; Ludivig, Philippe; Sajadi-Alamdari, Seyed Amin; Sanchez Lopez, Jose Luis; Olivares Mendez, Miguel Angel; Voos, Holger <br/> <br/>Abstract: Uncertain dynamic obstacles, such as pedestrians or vehicles, pose a major challenge for optimal robot navigation with safety guarantees. Previous work on motion planning has followed two main strategies to provide a safe bound on an obstacle's space: a polyhedron, such as a cuboid, or a nonlinear differentiable surface, such as an ellipsoid. The former approach relies on disjunctive programming, which has a relatively high computational cost that grows exponentially with the number of obstacles. The latter approach needs to be linearized locally to find a tractable evaluation of the chance constraints, which dramatically reduces the remaining free space and leads to over-conservative trajectories or even unfeasibility. In this work, we present a hybrid approach that eludes the pitfalls of both strategies while maintaining the original safety guarantees. The key idea consists in obtaining a safe differentiable approximation for the disjunctive chance constraints bounding the obstacles. The resulting nonlinear optimization problem is free of chance constraint linearization and disjunctive programming, and therefore, it can be efficiently solved to meet fast real-time requirements with multiple obstacles. We validate our approach through mathematical proof, simulation and real experiments with an aerial robot using nonlinear model predictive control to avoid pedestrians. <br/> <br/>Commentary: This paper has been accepted for publication in the IEEE Robotics and Automation Letters. Please cite the paper as: M. Castillo-Lopez, P. Ludivig, S. A. Sajadi-Alamdari, J. L. Sanchez-Lopez, M. A. Olivares-Mendez, H. Voos, "A Real-Time Approach for Chance-Constrained Motion Planning with Dynamic Obstacles", IEEE Robotics and Automation Letters (RA-L), 2020. Wed, 22 Jan 2020 14:08:19 GMT http://hdl.handle.net/10993/41859 Title: Conventional EO Satellites vs. CubeSats; FDL - AI flood detection onboard a Nano Satellite <br/> <br/>Author, co-author: Backes, Dietmar; Schumann, Guy; Teferle, Felix Norman Tue, 21 Jan 2020 17:26:14 GMT http://hdl.handle.net/10993/41845 Title: Flood Detection On Low Cost Orbital Hardware <br/> <br/>Author, co-author: Mateo-Garcia, Gonzalo; Oprea, Silviu; Smith, Lewis; Veitch-Michaelis, Josh; Schumann, Guy; Gal, Yarin; Baydin, AtılımGünes; Backes, Dietmar <br/> <br/>Abstract: Satellite imaging is a critical technology for monitoring and responding to natural disasters such as flooding. Despite the capabilities of modern satellites, there is still much to be desired from the perspective of first response organisations like UNICEF. Two main challenges are rapid access to data, and the ability to automatically identify flooded regions in images. We describe a prototypical flood segmentation system, identifying cloud, water and land, that could be deployed on a constellation of small satellites, performing processing on board to reduce downlink bandwidth by 2 orders of magnitude. We target PhiSat-1, part of the FSSCAT mission, which is planned to be launched by the European Space Agency (ESA) near the start of 2020 as a proof of concept for this new technology. Tue, 21 Jan 2020 15:52:51 GMT http://hdl.handle.net/10993/41749 Title: Non-localised contact between beams with circular and elliptical cross-sections <br/> <br/>Author, co-author: Magliulo, Marco; Lengiewicz, Jakub; Zilian, Andreas; Beex, Lars <br/> <br/>Abstract: The key novelty of this contribution is a dedicated technique to e fficiently determine the distance (gap) function between parallel or almost parallel beams with circular and elliptical cross-sections. The technique consists of parametrizing the surfaces of the two beams in contact, fixing a point on the centroid line of one of the beams and searching for a constrained minimum distance between the surfaces (two variants are investigated). The resulting unilateral (frictionless) contact condition is then enforced with the Penalty method, which introduces compliance to the, otherwise rigid, beams' cross-sections. Two contact integration schemes are considered: the conventional slave-master approach (which is biased as the contact virtual work is only integrated over the slave surface) and the so-called two-half-pass approach (which is unbiased as the contact virtual work is integrated over the two contacting surfaces). Details of the finite element formulation which is suitably implemented using Automatic Di fferentiation techniques are presented. A set of numerical experiments shows the overall performance of the framework and allows a quantitative comparison of the investigated variants. Mon, 20 Jan 2020 15:34:00 GMT http://hdl.handle.net/10993/41663 Title: Safer UAV Piloting: A Robust Sense-and-Avoid Solution for Remotely Piloted Quadrotor UAVs in Complex Environments <br/> <br/>Author, co-author: Wang, Min; Voos, Holger <br/> <br/>Abstract: Current commercial UAVs are to a large extent remotely piloted by amateur human pilots. Due to lack of teleoperation experience or skills, they often drive the UAVs into collision. Therefore, in order to ensure safety of the UAV as well as its surroundings, it is necessary for the UAV to boast the capability of detecting emergency situation and acting on its own when facing imminent threat. However, the majority of UAVs currently available in the market are not equipped with such capability. To fill in the gap, in this paper we present a complete sense-and-avoid solution for assisting unskilled pilots in ensuring a safe flight. Particularly, we propose a novel nonlinear vehicle control system which takes into account of sensor characteristics, an emergency evaluation policy and a novel optimization-based avoidance control strategy. The effectiveness of the proposed approach is demonstrated and validated in simulation with multiple moving objects. Fri, 17 Jan 2020 17:09:28 GMT http://hdl.handle.net/10993/40777 Title: A stochastic Galerkin cell-based smoothed finite element method (SGCS-FEM) <br/> <br/>Author, co-author: Mathew, Tittu; Beex, Lars; Bordas, Stéphane; Natarajan, Sundararajan <br/> <br/>Abstract: In this paper, the cell based smoothed finite element method is extended to solve stochastic partial diff erential equations with uncertain input parameters. The spatial field of Young's moduli and the corresponding stochastic results are represented by Karhunen-Lo eve expansion and polynomial chaos expansion, respectively. The Young's Modulus of structure is considered to be random for stochastic static as well as free vibration problems. Mathematical expressions and the solution procedure are articulated in detail to evaluate the statistical characteristics of responses in terms of static displacements and free vibration frequencies. The feasibility and eff ectiveness of the proposed SGCS-FEM method in terms of accuracy and lower requirement on the mesh size in the solution domain over that of conventional FEM for stochastic problems are demonstrated by carefully chosen numerical examples. From the numerical study, it is inferred that the proposed framework is computationally less demanding without compromising accuracy. Sat, 26 Oct 2019 03:30:02 GMT http://hdl.handle.net/10993/40625 Title: EFFECTS OF MULTIPLE OSCILLATOR PHASE NOISE IN PRECODING PERFORMANCE <br/> <br/>Author, co-author: Martinez Marrero, Liz <br/> <br/>Abstract: Satellite Precoding is a promising technique to meet the target data rates of the future high throughput satellite systems and the costs per bit as required by 5G applications and networks, but it requires strict synchronization among the transmitted waveforms, in addition to accurate channel state information. Most of the published work about this topic consider ideal oscillators, but in practice, the output of an oscillator is not a single spectral line at the nominal frequency. This paper proposes a model for the oscillator phase noise and analyzes the resulting received signal to interference plus noise ratio (SNIR) in a satellite communication system using Precoding. Simulations of a communication satellite system with a two-beam transponder and two receivers were performed to compute the effective SNIR. This work uses a simulator which also considers practical impairments such as time misalignment, errors in the channel state information, interference, thermal noise and phase noise masks for satellite oscillators. The Precoding methods used for the analysis are Zero Forcing (ZF) and Minimum Mean Square Error (MMSE). The obtained results prove that there is a degradation in the performance due to the use of independent oscillators but this effect is compensated by the precoding matrix. Mon, 07 Oct 2019 08:32:26 GMT http://hdl.handle.net/10993/40513 Title: Adaptive smoothed stable extended finite element method for weak discontinuities for finite elasticity <br/> <br/>Author, co-author: Jansari, Chintan; Natarajan, Sundararajan; Beex, Lars; Kannan, Krishna <br/> <br/>Abstract: In this paper, we propose a smoothed stable extended finite element method (S2XFEM) by combining the strain smoothing with the stable extended finite element method (SXFEM) to efficiently treat inclusions and/or voids in hyperelastic matrix materials. The interface geometries are implicitly represented through level sets and a geometry based error indicator is used to resolve the geometry. For the unknown fields, the mesh is refined based on a recovery based error indicator combined with a quadtree decomposition guarantee the method’s accuracy with respect to the computational costs. Elements with hanging nodes (due to the quadtree meshes) are treated as polygonal elements with mean value coordinates as the basis functions. The accuracy and the convergence properties are compared to similar approaches for several numerical examples. The examples indicate that S2XFEM is computationally the most efficient without compromising the accuracy. Wed, 25 Sep 2019 13:36:19 GMT http://hdl.handle.net/10993/40339 Title: A comparison between conventional Earth Observation Satellites and CubeSats; Requirements, Capabilities and Data Quality <br/> <br/>Author, co-author: Backes, Dietmar; Hassani, Saif Alislam; Teferle, Felix Norman; Schumann, Guy <br/> <br/>Abstract: From its early beginning as an educational tool in 1999, cubesats have evolved into a popular platform for technology demonstrations and scientific instruments. Ideas and innovations sparked from an enthusiastic community led to the development of new Earth Observation (EO) technology concepts based on large constellations of satellites with high-resolution optical imagers previously considered as infeasible. Probably the most significant constellation today is deployed by Planet who are currently operating a fleet larger than 120 3U Dove satellites, which provide an imaging service with up to 3m Ground Sample Distance (GSD). The number of low-cost EO Cubesat systems is constantly increasing. However, for a number of reasons there still seems to be a reluctance to use such data for many EO applications. A better understanding of the capabilities of the current generation of small Cubesats compared to the traditional well-established bigger operational missions of high and medium resolution EO satellites is required. What are the critical capabilities and quality indicators? Due to the limited size and weight of Cubesats, critical system components, e.g. for navigation and communication, always compete with operational payloads such as optical camera/sensor systems. A functional EO system requires balanced payload, which provides adequate navigational capabilities, that match the requirements of the optical imagers (camera) deployed with the system. This study reviews the current performance and capabilities of Cubesats for optical EO and compares them to the capabilities of conventional, dedicated high and medium resolution EO systems. We summarise key performance parameters and quality indicators to evaluate the difference between the systems. An empirical study compares recent very high-resolution (VHR) imagery from big EO satellite missions with available images from Cubesats for the use case in disaster monitoring. Small and agile Nanosatellites or Cubesats already show remarkable performance. Although it is not expected that their performance and capability will match those of current bigger EO satellite missions, they are expected to provide a valuable tool for EO and remote sensing, in particular for downstream industry applications. Thu, 12 Sep 2019 16:15:44 GMT http://hdl.handle.net/10993/40329 Title: Contact between shear-deformable beams with elliptical cross-sections <br/> <br/>Author, co-author: Magliulo, Marco; Zilian, Andreas; Beex, Lars <br/> <br/>Abstract: Slender constituents are present in many structures and materials. In associated mechanical models, each slender constituent is often described with a beam. Contact between beams is essential to incorporate in mechanical models, but associated contact frameworks are only demonstrated to work for beams with circular cross-sections. Only two studies have shown the ability to treat contact between beams with elliptical cross-sections, but those frameworks are limited to point-wise contact, which narrows their applicability. This contribution presents initial results of a framework for shear-deformable beams with elliptical cross-sections if contact occurs along a line or at an area (instead of at a point). This is achieved by integrating a penalty potential over one of the beams’ surfaces. Simo-Reissner Geometrically Exact Beam (GEB) elements are employed to discretise each beam. As the surface of an assembly of such beam elements is discontinuous, a smoothed surface is introduced to formulate the contact kinematics. This enables the treatment of contact for large sliding displacements and substantial deformations. Thu, 12 Sep 2019 03:30:01 GMT http://hdl.handle.net/10993/40181 Title: A Bayesian framework to identify random parameter fields based on the copula theorem and Gaussian fields: Application to polycrystalline materials <br/> <br/>Author, co-author: Rappel, Hussein; Wu, Ling; Noels, Ludovic; Beex, Lars <br/> <br/>Abstract: For many models of solids, we frequently assume that the material parameters do not vary in space, nor that they vary from one product realization to another. If the length scale of the application approaches the length scale of the micro-structure however, spatially fluctuating parameter fi elds (which vary from one realization of the fi eld to another) can be incorporated to make the model capture the stochasticity of the underlying micro-structure. Randomly fluctuating parameter fields are often described as Gaussian fields. Gaussian fi elds however assume that the probability density function of a material parameter at a given location is a univariate Gaussian distribution. This entails for instance that negative parameter values can be realized, whereas most material parameters have physical bounds (e.g. the Young's modulus cannot be negative). In this contribution, randomly fluctuating parameter fi elds are therefore described using the copula theorem and Gaussian fi elds, which allow di fferent types of univariate marginal distributions to be incorporated, but with the same correlation structure as Gaussian fields. It is convenient to keep the Gaussian correlation structure, as it allows us to draw samples from Gaussian fi elds and transform them into the new random fields. The bene fit of this approach is that any type of univariate marginal distribution can be incorporated. If the selected univariate marginal distribution has bounds, unphysical material parameter values will never be realized. We then use Bayesian inference to identify the distribution parameters (which govern the random fi eld). Bayesian inference regards the parameters that are to be identi fied as random variables and requires a user-defi ned prior distribution of the parameters to which the observations are inferred. For the homogenized Young's modulus of a columnar polycrystalline material of interest in this study, the results show that with a relatively wide prior (i.e. a prior distribution without strong assumptions), a single specimen is su ciffient to accurately recover the distribution parameter values. Wed, 28 Aug 2019 09:26:20 GMT http://hdl.handle.net/10993/40097 Title: Fusing the Seth-Hill strain tensors to fit compressible elastic material responses in the nonlinear regime <br/> <br/>Author, co-author: Beex, Lars <br/> <br/>Abstract: Strain energy densities based on the Seth-Hill strain tensors are often used to describe the hyperelastic mechanical behaviours of isotropic, transversely isotropic and orthotropic materials for relatively large deformations. Since one parameter distinguishes which strain tensor of the Seth-Hill family is used, one has in theory the possibility to t the material response in the nonlinear regime. Most often for compressible deformations however, this parameter is selected such that the Hencky strain tensor is recovered, because it yields rather physical stress-strain responses. Hence, the response in the nonlinear regime is in practise not often tailored to match experimental data. To ensure that elastic responses in the nonlinear regime can more accurately be controlled, this contribution proposes three generalisations that combine several Seth-Hill strain tensors. The generalisations are formulated such that the stress-strain responses for in finitesimal deformations remain unchanged. Consequently, the identifi cation of the Young's moduli, Poisson's ratios and shear moduli is not a ffected. 3D fi nite element simulations are performed for isotropy and orthotropy, with an emphasis on the identifi cation of the new material parameters. Mon, 12 Aug 2019 11:11:57 GMT http://hdl.handle.net/10993/39666 Title: Multiscale fracture: a natural connection between reduced order models and homogenisation <br/> <br/>Author, co-author: Bordas, Stéphane; Beex, Lars; Chen, Li; Vijayaraghavan, Soumianarayanan; Kerfriden, Pierre; Goury, Olivier; Massart, Thierry; Noels, Ludovic Thu, 13 Jun 2019 03:30:07 GMT http://hdl.handle.net/10993/39279 Title: Bayesian Identification of Mean-Field Homogenization model parameters and uncertain matrix behavior in non-aligned short fiber composites <br/> <br/>Author, co-author: Mahamedou, Mohamed; Zulueta Uriondo, Kepa; Chung, Chi Nghia; Rappel, Hussein; Beex, Lars; Adam, Laurent; Arriaga, Aitor; Major, Zoltan; Wu, Ling; Noels, Ludovic <br/> <br/>Abstract: We present a stochastic approach combining Bayesian Inference (BI) with homogenization theories in order to identify, on the one hand, the parameters inherent to the model assumptions and, on the other hand, the composite material constituents behaviors, including their variability. In particular, we characterize the model parameters of a Mean-Field Homogenization (MFH) model and the elastic matrix behavior, including the inherent dispersion in its Young's modulus, of non-aligned Short Fibers Reinforced Polymer (SFRP) composites. The inference is achieved by considering as observations experimental tests conducted at the SFRP composite coupons level. The inferred model and material law parameters can in turn be used in Mean-Field Homogenization (MFH)-based multi-scale simulations and can predict the confidence range of the composite material responses. Thu, 04 Apr 2019 09:32:35 GMT http://hdl.handle.net/10993/38359 Title: Aircraft landing gear thermo-tribomechanical model development <br/> <br/>Author, co-author: Heirendt, Laurent <br/> <br/>Abstract: A transient numerical model for studying the thermo-tribomechanical behavior of an aircraft landing gear is presented. The study reveals the major heat sources and heat sinks that impact the characteristic thermal behavior of the landing gear shock absorber. The severe in-service performance degradation and reported structural damage can be explained as a consequence of the heat generated by the high drag loads induced by rough runways on the bearings, and by the high sliding velocities of the piston. A conclusive model may lead to improved landing gear performance once the transient process of heat generation in a phase-changing grease-lubricated lower bearing is fundamentally understood. A novel tribotopological lubrication theory is derived in order to take into account all distinct physical phases of the non-Newtonian Bingham lubricant. The governing equations are solved using a hybrid numerical solver that is optimized for numerical efficiency and fast convergence. The proposed framework is validated against existing theories and results, and it demonstrates accurate predictions of the thermal performance of the landing gear. Strategies to passively optimize the lower bearing lubrication mechanism are further suggested in order to achieve optimal thermal performance of future aircraft landing gear. Thu, 17 Jan 2019 19:10:02 GMT http://hdl.handle.net/10993/38358 Title: Aircraft landing gear greased slider bearing steady-state thermo-elastohydrodynamic concept model <br/> <br/>Author, co-author: Heirendt, Laurent; Hugh, Liu; Wang, Phillip <br/> <br/>Abstract: This paper presents a steady-state concept model for studying the thermal behavior of a greased aircraft landing gear lower slider bearing. Structural damage has been reported as a consequence of excessive heat generated by the high loads induced by rough runways on the bearings, and by the high sliding velocities of the piston. The goal of the model is to enable fundamental understanding of the frictional heat generation. The governing equations are adapted for grease flow and special attention is given to the underlying algorithm of the developed numerical framework used to efficiently solve the governing equations. The developed numerical code is validated against existing results. Numerical results indicate fundamental differences in fluid flow behavior between greased and oil-lubricated bearings. Thu, 17 Jan 2019 18:54:22 GMT http://hdl.handle.net/10993/38357 Title: Lubrication mechanism design for aircraft landing gear bearings <br/> <br/>Author, co-author: Heirendt, Laurent; Hugh, Hugh H.T.; Wang, Phillip <br/> <br/>Abstract: A shock absorber with lubricated bearings for an aircraft landing gear includes a piston that is received in a cylinder, and an upper bearing fixed to the piston that slidably engages an inner surface of the cylinder. A lower bearing extends inwardly from a lower portion of the cylinder and engages an outer surface of the piston. The lower bearing has a center axis and defines an annular bearing surface configured to slidably engage the piston outer surface. The annular bearing surface has a first portion that extends circumferentially more than one hundred eighty degrees about the center axis at a constant radius, defining a circular annular segment. A second portion closes the circular annular segment and defines a shallow channel or pocket in the annular bearing surface. In some embodiments the lower bearing further comprises oppositely disposed frustoconical thrust portions. Thu, 17 Jan 2019 18:45:31 GMT http://hdl.handle.net/10993/38355 Title: Aircraft Landing Gear Thermo-Tribomechanical Model and Sensitivity Study <br/> <br/>Author, co-author: Heirendt, Laurent; Wang, Phillip; Liu, Hugh H.T. <br/> <br/>Abstract: A methodology for characterizing the thermo-tribomechanical response of an aircraft landing gear shock absorber is presented. Structural damage has been reported as a consequence of heat generated by high loads induced by rough runways on the shock absorber bearings and by the high sliding velocities of the shock absorber piston. Therefore, a model that reveals the characteristics of the thermal behavior, and identifies heat sources and sinks in the landing gear shock absorber is developed. The thermo-tribomechanical model framework is presented with representative development of each component. In addition, a sensitivity study of the maximum heat flux to variations of key input parameters is investigated. The numerical results indicate that the runway amplitude dominates the landing gear thermal response. Thu, 17 Jan 2019 17:48:48 GMT http://hdl.handle.net/10993/38138 Title: Robust Online Obstacle Detection and Tracking for Collision-free Navigation of Multirotor UAVs in Complex Environments <br/> <br/>Author, co-author: Wang, Min; Voos, Holger; Su, Daobilige <br/> <br/>Abstract: Object detection and tracking is a challenging task, especially for unmanned aerial robots in complex environments where both static and dynamic objects are present. It is, however, essential for ensuring safety of the robot during navigation in such environments. In this work we present a practical online approach which is based on a 2D LIDAR. Unlike common approaches in the literature of modeling the environment as 2D or 3D occupancy grids, our approach offers a fast and robust method to represent the objects in the environment in a compact form, which is significantly more efficient in terms of both memory and computation in comparison with the former. Our approach is also capable of classifying objects into categories such as static and dynamic, and tracking dynamic objects as well as estimating their velocities with reasonable accuracy. Wed, 09 Jan 2019 10:29:48 GMT http://hdl.handle.net/10993/37929 Title: Estimating fibres' material parameter distributions from limited data with the help of Bayesian inference <br/> <br/>Author, co-author: Rappel, Hussein; Beex, Lars <br/> <br/>Abstract: Numerous materials are essentially structures of discrete fibres, yarns or struts. Considering these materials at their discrete scale, one may distinguish two types of intrinsic randomness that affect the structural behaviours of these discrete structures: geometrical randomness and material randomness. Identifying the material randomness is an experimentally demanding task, because many small fibres, yarns or struts need to be tested, which are not easy to handle. To avoid the testing of hundreds of constituents, this contribution proposes an identification approach that only requires a few dozen of constituents to be tested (we use twenty to be exact). The identification approach is applied to articially generated measurements, so that the identified values can be compared to the true values. Another question this contribution aims to answer is how precise the material randomness needs to be identified, if the geometrical randomness will also influence the macroscale behaviour of these discrete networks. We therefore also study the effect of the identified material randomness to that of the actual material randomness for three types of structures; each with an increasing level of geometrical randomness. Fri, 04 Jan 2019 14:50:14 GMT