A review of path following control strategies for autonomous robotic vehicles: Theory,simulations, and experiments

This article presents an in-depth review of path following control strategies that are applicable to a wide range class of marine, ground, and aerial autonomous robotic vehicles. From a control system standpoint, path following can be formulated as the problem of stabilizing a path following error system that describes the dynamics of position and possibly orientation errors of a vehicle with respect to a path, with the errors defined in an appropriate reference frame. In spite of the large variety of path following methods described in the literature we show that, in principle, most of them can be categorized in two groups: stabilization of the path following error system expressed either in the vehicle's body frame or in a frame attached to a “reference point” moving along the path, such as a Frenet-Serret (F-S) frame or a Parallel Transport (P-T) frame. With this observation, we provide a unified formulation that is simple but general enough to cover many methods available in the literature. We then discuss the advantages and disadvantages of each method, comparing them from the design and implementation standpoint. We further show experimental results of the path following methods obtained from field trials testing with under-actuated and over-actuated autonomous marine vehicles. In addition, we introduce open-source Matlab and Gazebo/ROS simulation toolboxes that are helpful in testing path following methods before their integration in the combined guidance, navigation, and control systems of autonomous vehicles.     

Bayesian non-linear regression models with skew-elliptical errors: Applications to the classification of longitudinal profiles

Typically, the fundamental assumption in non-linear regression models is the normality of the errors. Even though this model offers great flexibility for modeling these effects, it suffers from the same lack of robustness against departures from distributional assumptions as other statistical models based on the Gaussian distribution. It is of practical interest, therefore, to study non-linear models which are less sensitive to departures from normality, as well as related assumptions. Thus the current methods proposed for linear regression models need to be extended to non-linear regression models. This paper discusses non-linear regression models for longitudinal data with errors that follow a skew-elliptical distribution. Additionally, we discuss Bayesian statistical methods for the classification of observations into two or more groups based on skew-models for non-linear longitudinal profiles. Parameter estimation for a discriminant model that classifies individuals into distinct predefined groups or populations uses appropriate posterior simulation schemes. The methods are illustrated with data from a study involving 173 pregnant women. The main objective in this study is to predict normal versus abnormal pregnancy outcomes from beta human chorionic gonadotropin data available at early stages of pregnancy.     

Segmentation and quantification of COVID-19 infections in CT using pulmonary vessels extraction and deep learning

Multimedia Tools and Applications - At the end of 2019, the World Health Organization (WHO) reported pneumonia that started in Wuhan, China, as a global emergency problem. Researchers quickly...     

Distributed attribute-based access control system using permissioned blockchain

World Wide Web - Auditing provides essential security control in computer systems by keeping track of all access attempts, including both legitimate and illegal access attempts. This phase can be...     

Studies on grafting of cellulosic materials isolated from Agave lechuguilla and fourcroydes

The grafting reaction of styrene onto cellulosic material isolated from Agave lechuguilla and fourcroydes, using the initiator ceric ammonium nitrate with nitric acid, under variation of concentration of components, reaction conditions, and pretreatment of starting material with aqueous NaOH was investigated. By determination of the maximal load it was revealed that a graft yield of about 6% gave products with the best mechanical properties, although graft yields of up to 15% can be realized. The samples were characterized by means of FTIR spectroscopy, DSC measurements, and SEM. Preliminary studies show that the grafted materials are thermally more stable compared to the starting material and that they are biodegradable.     

Developing turbulent heat transfer in a 360° bend of square cross section

This paper reports the computational results on the developing turbulent heat transfer in a 360° bend of square cross section. The centrifugal force acting on the fluid in a bend flow induces strong cross-stream motion. It was found from the computations that the counterrotating vortex pair caused by the centrifugal force are broken into a multicell pattern after the θ = 90° station of the bend and the continuous arising and ceasing of the vortices directly affects the variation of heat transfer through the walls of the bend. Particular attention was paid to the developing process of the vortices because they exert the most significant effects on the convective heat transfer through the bend walls. A low Reynolds number second moment turbulence closure was employed to simulate the near wall turbulence behaviors of the bend. © 1999 Scripta Technica, Heat Trans Asian Res, 28(2): 77–88, 1999     

Investigation of the structure and properties of polyisobutylene-based telechelic ionomers of narrow molecular weight distribution. I.

The solid state morphology of recently developed sulfonated polyisobutylene (PIB) telechelic ionomers with narrow molecular weight distribution (MWD) (M¯_w/M¯_n ≈ 1.15) was investigated. A small angle X-ray scattering (SAXS) peak often associated with the aggregation of the ionic species in the bulk, as well as a secondary peak, have been observed in the narrow distribution sulfonated PIB telechelic ionomers for the first time. Ionomers of difunctional and tri-arm architecture at several number average molecular weights (M¯_n) with one of several counterions were investigated and the preparation method was also considered in terms of observed SAXS behavior. Compression-molded films made from these narrow MWD telechelics were examined in detail using SAXS. Primary and secondary peaks were observed in the slit-smeared SAXS profiles up to an M¯_n of 27 kg/mol. The ratio of the interdomain spacings derived from these peaks (ca. 2 : 1) suggests cylindrical or lamellar ordering in the morphology in both difunctional and tri-arm ionomers. Pinhole SAXS images showed no azimuthal dependence in the scattering pattern and thus, this ordering is on a local scale. It was found that the counterion has an inconsistent effect on the smeared interdomain spacing associated with the SAXS peak. The smeared interdomain spacings were systematic with respect to architecture and M¯_n for solution cast Cs⁺ ionomers. The difunctional telechelics exhibited higher smeared interdomain spacings than tri-arms of comparable M¯_n and unlike the compression-molded films, the smeared interdomain spacings of the solution cast Cs⁺ telechelic ionomers increased systematically with increasing M¯_n. Even a broad MWD (M¯_n/M¯_n ≈ 1.8) tri-arm telechelic ionomer M¯_n of 20.6 kg/mol exhibited a diffuse primary peak in SAXS when solution-cast. However, it exhibited no peak when compression-molded. © 1997 John Wiley & Sons, Inc.     

Conductivity variation induced by solvent swelling of an elastomer–carbon black–graphite composite

The behavior of a polybutadiene-carbon black composite entering into contact with organic solvents and gasoline was investigated. The composite used has a conductivity of 3.0 ± 0.1 Ω⁻¹ cm⁻¹. However, if it comes into contact with organic solvents (gasoline, for instance) the matrix absorbs them and consequently swells. This swelling causes the separation of the carbon particles and the concomitant diminution of the composite conductivity. During the tests performed, the resistivity of the composite grows exponentially with the exposure time to solvents. Typically, the material samples show a reduction of approximately 30% of its initial conductivity after only 1.5 min of exposure to solvents. Also, it was observed that the rate at which the conductivity decreases is related to the chemical nature of the solvent used in the test. To model the drop on composite conductivity induced by solvent swelling we use an effective media percolation approach. This approach was adapted to the needs of our experiments by modifying the definition of one of its main parameters (the critical volume of the low-conductivity fraction). The experimental data were successfully described by this model. Finally, the test performed shows that this composite is a very promising material that can be employed, for example, in various security and control devices to warning of accidental organic solvent or hydrocarbon leaks in pipelines or containers of chemical industries and refineries. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 2221–2232, 1997     

Computational Characterization of Mechanical,Hemodynamic, and Surface Interaction Conditions: Role of Protein Adsorption on the Regenerative Response of TEVGs

Currently available small diameter vascular grafts (<6 mm) present several long-term limitations, which has prevented their full clinical implementation. Computational modeling and simulation emerge as tools to study and optimize the rational design of small diameter tissue engineered vascular grafts (TEVG). This study aims to model the correlation between mechanical-hemodynamic-biochemical variables on protein adsorption over TEVG and their regenerative potential. To understand mechanical-hemodynamic variables, two-way Fluid-Structure Interaction (FSI) computational models of novel TEVGs were developed in ANSYS Fluent 2019R3^® and ANSYS Transient Structural^® software. Experimental pulsatile pressure was included as an UDF into the models. TEVG mechanical properties were obtained from tensile strength tests, under the ISO7198:2016, for novel TEVGs. Subsequently, a kinetic model, linked to previously obtained velocity profiles, of the protein-surface interaction between albumin and fibrinogen, and the intima layer of the TEVGs, was implemented in COMSOL Multiphysics 5.3^®. TEVG wall properties appear critical to understand flow and protein adsorption under hemodynamic stimuli. In addition, the kinetic model under flow conditions revealed that size and concentration are the main parameters to trigger protein adsorption on TEVGs. The computational models provide a robust platform to study multiparametrically the performance of TEVGs in terms of protein adsorption and their regenerative potential.