The Influence of Powder Porosity on the Bonding Mechanism at the Impact of Thermally Sprayed Solid Particles

High-velocity oxy-fuel (HVOF) thermal spraying can generate dense depositions without melting the powders during spraying. Our previous study showed that most HVOF-sprayed particles are in solid state prior to impact on the substrate. The deposition of solid particles requires sufficient deformation of the particles as a result of a high impact. This report is a continuation of our previous work to study the bonding mechanism for thermally sprayed solid particles. The same hard material, WC-Co powder, is studied by considering the porosity inside the particles. The detailed deposition mechanism is examined by dynamically tracking the particle impingement using finite element analysis (FEA) models. The results confirm that the deposition of high-speed solid particles is caused mainly by the particle deformation and further implies that deformation is enhanced with increase in porosity alone. Therefore, a possible way to increase the deposition efficiency of hard cermet coating could be to use a properly designed porous powder.     

Compensation of teleoperator modeling uncertainties with a sliding mode controller

The response of a previously proposed teleoperator system, which was not based on the traditional notion of telepresence but on an original form of kinesthetic feedback, is tested under modeling errors of up to 50%. The original system is enhanced by replacing the previous controller of each robot with a sliding mode robust one. A two-stage approach is employed: a desired trajectory is first generated and then tracked by the controller. The difficulties incurred for the man/machine cooperation are discussed. This extension of the original teleoperator architecture is tested through simulations and compared with a twin system based on neural controllers, presented elsewhere.     

Hydraulic Roughness of Breakup Ice Jams

Lack of field data on the thickness and roughness of river ice jams inhibits reliable calibration of ice jam models and can result in erroneous evaluation of important physical parameters. Detailed thickness data that have been obtained on several ice jams by means of a remote profiling technique reveal a highly irregular underside. Statistical analysis points to a normal probability distribution of the irregularities and allows direct assessment of the absolute roughness of each jam. The latter varies linearly with the average jam thickness, at least to a thickness value of 3 m, and is shown to be consistent with previously reported roughness values deduced from purely hydraulic considerations. Using the present findings in conjunction with past data and modelling experience, simple equations are developed to compute composite-flow resistance characteristics in terms of jam thickness and flow depth.     

Stress concentrations in multi-planar welded CHS XX-connections

The present paper concerns the prediction of stress concentration factors (SCFs) of multi-planar welded tubular (CHS) XX-joints in steel structures. SCFs are used in the fatigue design of tubular joints for the calculation of the joint hot-spot stress. Extensive computational research employing finite elements has been conducted, supported by experimental evidence, which resulted in a large SCF data base for multi-planar joints. Using this data base, a simple and efficient methodology has been developed to predict stress concentrations in XX-joints under various loading conditions. The SCF results are presented in the form of parametric equations and diagrams to be used by design engineers in everyday practice. This approach is aimed towards a simple and efficient tubular joint design of tubular structures subjected to repeated loading.     

Conflict resolution in AGV systems

Currently, conflict-free routing in AGV systems is established by means of one of the following three approaches: (i) the problem elimination through the adoption of a segmented path flow or tandem queue configuration; (ii) the identification of imminent collisions through forward sensing and their aversion through vehicle backtracking and/or rerouting; or (iii) the imposition of zone control and extensive route pre-planning, typically based on deterministic timing of the vehicle traveling and docking stages. Among these three approaches, the segmented path flow-based approach presents the highest robustness to the system stochas-ticities/randomness, but at the cost of restricted vehicle routings and the need for complicated handling operations. This paper proposes an alternative conflict resolution strategy that will ensure robust AGV conflict resolution, while maintaining the operational flexibility provided by free vehicle travel on arbitrarily structured guidepath networks. Specifically, the approach advocated in this paper also employs zone control, but it determines vehicle routes incrementally, one zone at a time. Routing decisions are the result of a sequence of safety and performance considerations, with the former being primarily based on structural/logical rather than timing aspects of the system behavior. The resulting control problem is characterized as the AGV structural control. After defining the notion of AGV structural control, the paper proceeds to the formal characterization and analysis of the problem, and to the development of a structural control policy appropriate for the class of AGV resource allocation systems. The paper concludes with some discussion on the accommodation of emerging AGV operational features in the proposed modeling and analysis framework, and the integration of AGV structural control with the broader control of material-flow among the shop-floor workstations.     

A Novel Scheme for Human-Friendly and Time-Delays Robust Neuropredictive Teleoperation

A novel Neuropredictive Teleoperation (NPT) Scheme is presented. The design results from two key ideas: the exploitation of the measured or estimated neural input to the human arm or its electromyograph (EMG) as the system input and the employment of a predictor of the arm movement, based on this neural signal and an arm model, to compensate for time delays in the system. Although a multitude of such models, as well as measuring devices for the neural signals and the EMG, have been proposed, current telemanipulator research has only been considering highly simplified arm models. In the present design, the bilateral constraint that the master and slave are simultaneously compliant to each other's state (equal positions and forces) is abandoned, thus obtaining a simple to analyze succession of only locally controlled modules, and a robustness to time delays of up to 500 ms. The proposed designs were inspired by well established physiological evidence that the brain, rather than controlling the movement on-line, programs the arm with an action plan of a complete movement, which is then executed largely in open loop, regulated only by local reflex loops. As a model of the human arm the well-established Stark model is employed, whose mathematical representation is modified to make it suitable for an engineering application. The proposed scheme is however valid for any arm model. BIBO-stability and passivity results for a variety of local control laws are reported. Simulation results and comparisons with traditional designs also highlight the advantages of the proposed design.     

Observer-Based Optimal Control of Flexible Stewart Parallel Robots

The design and simulation of two optimal control schemes for a parallel flexible link manipulator of the Stewart type is presented. The first control scheme combines a nonlinear rigid model of the flexible manipulator with a linear rigid observer, whereas the second scheme uses a nonlinear flexible manipulator model with a linear flexible observer. The majority of the results available in the literature do not address the optimal control problem through the use of observers, as it is done in this paper, for the control of parallel robots. The simulation results have shown in both cases that indeed optimal state-obscrver-based control is a good candidate for controlling parallel-link manipulators in practice. As expected, the second scheme (flexible model plus flexible observer) gives better results than the first one, achieving faster trajectory tracking. The effects of white noises, applied forces, and zero gravity environment were taken under consideration.     

Cyberbullying‐Entrenched or Cyberbully‐Free Classrooms? A Class Network and Class Composition Approach

Cyberbullying threatens young people's well‐being. This study is one of the first to examine cyberbullying from a social network perspective. The class‐based friendship networks of 103 classes were reconstructed using peer‐nomination questionnaires. Closeness centralization and the global clustering coefficient were calculated using social network analysis and subsequently entered into a Poisson‐regression. The outcomes of these analyses show that in classes featured by high closeness centralization in the offline and online friendship network, more cyberbullying happens. Additionally, our analyses reveal that in classes featuring a high global clustering coefficient in the online network, less cyberbullying occurs. The composition of the class in terms of gender and ethnicity were not significantly associated with the number of cyberbullying incidents in class.