263 research outputs found
Operative management of isolated posterior cruciate ligament injuries improves stability and reduces the incidence of secondary osteoarthritis: a systematic review
No full textPurpose To evaluate the evidence for operative and non-operative management of isolated posterior cruciate ligament (PCL) injuries. Methods Using Pubmed, EMBASE and Cochrane databases, a systematic review was conducted of studies investigating the treatment of isolated PCL injuries published until July 2020. Quality assessment was performed with the Cochrane risk of bias tool (level I), the Newcastle-Ottowa Scale (level II-III) and the National Institute of Health quality assessment tool (level IV). Clinical outcome measures included residual laxity, return to sports, patient-reported outcome measures, subsequent articular degeneration and complications. Results Twenty-seven studies [23 case series, 2 case-control, 1 cohort study and 1 randomized controlled trial (RCT)] including 5197 patients (5199 knees) with a mean age of 29.5 +/- 3.6 years (range 15-68) fulfilled the study requirements. Significantly less residual laxity was found after posterior cruciate ligament reconstruction (PCLR) compared to non-operative management (3.43 vs. 5.47 mm, CI: 1.84-2.23, p < 0.001). Both treatment modalities yielded satisfying functional outcomes and a high return to sports (64-77%, mean: 70.3, CI: 67.8-72.2). Osteoarthritis (OA) occurred less frequently following PCLR (21.5 vs. 44.1%, p < 0.001). Conclusion In the absence of level I RCTs, this systematic review suggests that surgical management for selected isolated PCL injuries is a reasonable option to consider, especially when the surgeon aims at minimizing residual laxity and presumably secondary osteoarthritis
The Power of Ideas: Esotericism, Historicism, and the Limits of Discourse
No full textThis article is a response to the reviews by Giovanni Filoramo, Olav Hammer, Bernd-Christian Otto, Marco Pasi, and Michael Stausberg of Wouter J. Hanegraaff's book Esotericism and the Academy: Rejected Knowledge in Western Culture. The author attempts to clarify his view of the relation between history and theory, and between intellectual history and discourse theory in particular; he provides a summary of his motivations for writing the book and of its main argument; he concludes by addressing a number of theoretical issues raised by the respondents
Een beeld van een kust: Inspiratie voor de Kustnota
No full textDe kust als spiegel / Maurits Groen De levende kust / Govert D. Geldof Integraal beleid : te kust en te keur? / Gerrit Hagelstein en Philip Idenburg De nieuwe kusten van Noordzee en IJsselmeer / Wouter van Dieren De kust als menselijke gradiënt / Reinier Jan Scheele Duinen in een levende kust / Helias Udo de Haas Het verlangen naar de kust : de kust als toeristisch-recreatief domein / Theo Beckers De kust over 25 jaar : een normatief gezichtspunt / Karel Soudijn Een beeld van kust / Anne van der Meidenkustnot
Centralized and decentralized control of structural vibration and sound radiation
No full textThis paper examines the performance of centralized and decentralized feedback controllers on a plate with multiple colocated velocity sensors and force actuators. The performance is measured by the reduction in either kinetic energy or sound radiation, when the plate is excited with a randomly distributed, white pressure field or colored noise. The trade-off between performance and control effort is examined for each case. The controllers examined are decentralized absolute velocity feedback, centralized absolute velocity feedback control and linear quadratic Gaussian (LQG) control. It is seen that, despite the fact that LQG control is a centralized, dynamic controller, there is little overall performance improvement in comparison to decentralized direct velocity feedback control if both are limited to the same control effort
Design of the European Lunar Penetrator (ELUPE) Descent Module Controller
No full textTo obtain unambiguous ground truth of water-ice residing in permanently shaded regions of the Moon and to characterise the local regolith, ESA considers a mission involving an instrumented penetrator implanted there by high-speed impact. Released into lunar orbit, the European Lunar Penetrator (ELUPE) Descent Module will autonomously traverse a controlled trajectory to its designated target. The associated attitude control problem involves highly nonlinear large-angle slew manoeuvres and unstable minor-axis spin manoeuvres. To establish a benchmark, a controller based on classical control techniques was designed, verified and tested in a simulation. Legacy control algorithms were implemented and extended. A thruster management function was developed to translate the control commands into thruster actions. For the simulator, accurate models of the descent module and its environment were created.A Monte Carlo simulation was run to determine the success rate of the ELUPE mission from a descent-and-landing perspective, and to assess the performance of the controller under off-nominal conditions. From the results, it was found that the success rate was 58.5% for a surface slope of 20°, and 74.2% for a slope of 10° or lower. Key factors affecting the success rate were identified to be the centre-of-mass offset and the solid rocket motor thrust misalignment angle. As further constraining these parameters would be unrealistic, it was recommended to modify the thrust curve of the solid rocket motor to improve the success rate.Analysis of the attack angle and the nutation angle just prior to impact, revealed their success criteria were met in 98.7% and 99.8% of all cases, respectively. These successful results confirmed the attitude control problem could be satisfactorily solved by a 'classical' controller. However, despite its good global performance, the proposed controller was found to also exhibit some serious shortcomings. For this reason, it was recommended to explore the possibilities for a different controller.To the best of the author's knowledge, this thesis represented the first known attempt at designing a comprehensive controller for a fully actuated, thruster-controlled penetrator mission targeted for an airless body. In addition, it was the first known study to provide insight into the feasibility and success rate of such a mission from a descent-and-landing perspective.Aerospace Engineerin
A Decentralized Recovery Method for Air Traffic Conflicts
No full textIn an effort to increase airspace capacity, new methods for the decentralization of air traffic control have been a topic for research since the early nineties. The Modified Voltage Potential is a method to provide decentralized conflict detection and resolution. Through this method, aircraft can fly direct routes while resolving conflicts in a distributed manner. Though successfully implemented in various studies, its behavior in conflicts where aircraft are flying near parallel tracks has raised concern. It was found that problems can arise during the recovery after the conflict resolution, the phase where an aircraft regards the conflict as resolved and reverts to its desired velocity. To improve the Modified Voltage Potential's behavior during recovery, a modification is proposed. Through the use of two criteria, aircraft evaluate whether a conflict is resolved and if they can revert to their initial or desired velocity. Through analysis of a state-based solution space and an experiment, this study finds that the proposed recovery method significantly improves the stability, efficiency, and safety of the MVP method.Aerospace Engineerin
ADS-B Signal Integrity and Security Verification Using a Coherent Software Defined Radio: Mitigation of the threat of maliciously injected signals in ADS-B networks
No full textAutomatic Dependent Surveillance – Broadcast (ADS-B) is an operational enhancement as part of next-generation air transportation systems in Air Traffic Control. It enables aircraft and airport vehicles to periodically broadcast the information from their on-board equipment, like their identification, GPS location, velocity, and intent. Compared to classical radar surveillance, the service implementation has increased the renewal time, reduced costs, and increased safety and accuracy already. Nowadays, Mode S 1090 Extended Squitter is the most predominant adopted technology ADS-B service implementation.However, the ATC system has not been developed with security in mind and is vulnerable to a number of different radio frequency attacks by malicious parties. ADS-B is planned for long-term use but lacks the minimal and necessary inherent security mechanisms.This study suggests a possible and cost-effective solution that improves the security and integrity of raw ADS-B signals by designing a tool which can verify and validate the low-level signal. In this study, in order to mitigate the threat of maliciously injected signals, a method is proposed where two variables of direction of arrival are independently determined using a multi-channel coherent receiver. First, a calculated angle using signal decoding and trigonometry and secondly, an estimated angle using phase relationships and spatial correlation. Finally, an integrity verification method has been proposed and successfully applied.Aerospace Engineerin
Het meten en schatten van de prestaties van een stedelijk wegennetwerk: Toepassing op Den Haag
No full textMet de ontwikkelde datafusiemethode, die snelheden met gewogen intensiteiten fuseert, is het mogelijk om in plaats van Fundamentele Diagrammen op snelwegen en Macroscopisch Fundamentele Diagrammen voor hele netwerken, ook op traject gebaseerde Macroscopisch Fundamentele Diagrammen door complex stedelijk gebied te verkrijgen. Met een traject specifiek gefit Macroscopisch Fundamenteel Diagram is het schatten van missende informatie over zowel intensiteiten als reistijden vervolgens mogelijk wanneer de andere grootheid bekend is. Vooral voor reistijden leidt dit tot relatief goede schattingen. In de beroepspraktijk blijkt daarnaast ook dat vooral het hebben van informatie over reistijden erg waardevol wordt gevonden. Deze kunnen namelijk worden gebruikt voor de verkeerskundige prestatie-indicator “Level Of Service gebaseerd op gemiddelde reistijd per kilometer, met toegevoegde standaarddeviatie van de reistijden die langer dan gemiddeld duren”. Deze indicator is uit een ontwikkelde lijst van getoetste indicatoren als meest geschikte maatstaf aanbevolen voor de beroepspraktijk om de verkeerskundige prestaties van een stedelijk wegennetwerk op een eenduidige en communiceerbare manier weer te geven. Hiermee leiden de resultaten van het onderzoek tot een beter en completer inzicht van de verkeerskundige prestaties van een stedelijk wegennetwerk, om deze vervolgens op een eenduidige en communiceerbare manier weer te kunnen geven
Redesign of a Tata Steel transfer chute with dust liberation problems
No full textDust liberation in transfer chutes has been a persistent problem in the bulk handling industry, with an increasing demand for solutions due to the tightening of environmental regulations. An investigation into the root causes of dust liberation and a chute redesign with improvements in dust liberation potential has been performed in this research. The biggest challenges of this redesign being that the case study chute is a multimaterial, movable chute with differing flow rates and a redirection of material by 90±. The main, root causes of dust liberation have been found to be material impact, air entrainment and compact containment of the granular flow. Measurements at the current transfer chute have been performed to investigate and quantify the problems of the case study chute. Direct measurement of dust in and around transfer chute is often circumvented to measure related issues such as material degradation and air flow measurements, since this makes localizing dust liberation sources and quantification difficult. Therefore, stopped-belt sampling has been performed to find material degradation in a case study chute at Tata Steel, given that impacts cause degradation as well as dust liberation. No conclusive evidence was found due to a lack of samples. Iron ore pellets and sinter were dynamically calibrated using an inclined surface wear tester and simulations in EDEM software were performed for 800 t/h and 1600 t/h flow rates. Three problem areas were found where impacts and bulk density increases provided indications of potential dust liberation. A redesign proposal that uses a hood and spoon concept with movable hood, where the cut-off of the movable chute head needed to be heightened to fit a spoon that can improve flow conditions, was proposed. Improvements in the identified problem areas were found in simulations of the redesign.Marine Technology | Transport Engineering and Logistic
Autonomous Maneuvering of a Waterborne Vehicle in Close Proximity of Obstacles
No full textIn the automotive industry, automation is on the rise and it increases safety while decreasing costs. Improved sensor performance and greater computing power steers the future of the shipping industry in the same direction. Avoiding obstacles in close proximity is one of the current challenges. This research provides a generic and open- source design guideline for mapping, path planning and control for autonomous sailing in an environment with obstacles in close proximity. For each of the three modules, several options are discussed and the best one is chosen. Mapping is applied in the form of mapping with positioning sensors in an occupancy grid map. The original map is devided into cells containing information about the probability that they represent an obstacle. The mapping algorithm is applied in several maps and with noise and compared with a simultanously localization and mapping (SLAM) method. The resulting maps are created within the path planning requirements. Path planning is performed by inflating the occupancy grid map from the mapping algorithm. The inflated map is sampled into an 8-connected grid matching the grid cell size. A Dijkstra algorithm is applied combined with an quadratic approximation in the costmap. A steepest descent method will return a continous and shortest path which is suitable for control and does not collide with the obstacles in the map. Proportional and integral is applied to steer the vessel along the path. The proportional controller is tuned based upon the look ahead distance and its maximum error with the original path. The controller is tuned using a block shaped path as a reference and evaluated in a path from the path planner. Both static errors like wind or current and Gaussian noise on the position estimation are included for validation. Finally the controller is able to maneuver the vessel safely from its starting point to its final destination The functionalities of the modules are indivividually demonstrated, and eventually interconnected into one system. Autonomous maneuvering of the complete system is then demonstrated in various simulated environ- ments and settings, including unknown areas and global planning challenges. The vessel is able to maneuver autonomously from the start to its goal given only the sensor data, its location and its goal position
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