1,721,236 research outputs found
Drag-Free Motion Control of Satellite for High-Precision Gravity Field Mapping
Full text linkHigh precision mapping of the geoid and the Earth's gravity field are of importance to a wide range of ongoing studies in areas like ocean circulation, solid Earth physics and ice sheet dynamics. Using a satellite in orbit around the Earth gives the opportunity to map the Earth's gravity field in 3 dimensions with much better accuracy and spatial resolution than ever accomplished. To reach the desired quality of measurements, the satellite must fly in a low Earth orbit where disturbances from atmospheric drag and the Earth's magnetic field will perturb the satellite's motion. These effects will compromise measurement accuracy, unless they are accurately compensated by on-board thrusters. The paper concerns the design of a control system to performing such delicate drag compensation. A six degrees-of-freedom model for the satellite is developed with the model including dynamics of the satellite, sensors, actuators and environmental disturbances to the required micro-Newton accuracy. A control system is designed to compensate the non-gravitational disturbances on the satellite in three axes using an H∞-design. Performance is validated against mission requirements. Keywords: Spacecraft Attitude and Orbit Control, Drag Compensation, Drag-free motion
Ship Roll Damping Control
Full text linkThe technical feasibility of roll motion control devices has been amply demonstrated for over 100 years. Performance, however, can still fall short of expectations because of difficulties associated with control system designs, which have proven to be far from trivial due to fundamental performance limitations and large variations of the spectral characteristics of wave-induced roll motion. This tutorial paper presents an account of the development of various ship roll motion control systems together with the challenges associated with their design. It discusses the assessment of performance and the applicability of different mathematical models, and it surveys the control methods that have been implemented and validated with full scale experiments. The paper also presents an outlook on what are believed to be potential areas of research within this topic
Dynamic Model for Thrust Generation of Marine Propellers
Full text linkMathematical models of propeller thrust and torque are traditionally based on steady state thrust and torque characteristics obtained in model basin or cavitation tunnel tests. Experimental results showed that these quasi steady state models do not accurately describe the transient phenomena in a thruster. A recently published dynamic model was based on the experimental observations. Describing zero advance speed conditions accurately, this model, however, does not work for a vessel at non- zero relative water speed. This paper derives a large signal dynamic model of propeller that includes the eects of transients in the ow over a wide range of operation. The results are essential for accurate thrust control in dynamic positioning and in underwater robotics
Ship Roll Motion Control
Full text linkThe technical feasibility of roll motion control devices has been amply demonstrated for over 100 years. Performance, however, can still fall short of expectations because of deciencies in control system designs, which have proven to be far from trivial due to fundamental performance limitations. This tutorial paper presents an account of the development of various ship roll motion control systems and the challenges associated with their design. The paper discusses how to assess performance, the applicability of dierent models, and control methods that have been applied in the past
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Control of Ship Roll Motion
No full textThe undesirable effects of roll motion of ships (rocking about the longitudinal axis) became noticeable in the mid-nineteenth century when significant changes were introduced to the design of ships as a result of sails being replaced by steam engines and the arrangement being changed from broad to narrow hulls. The combination of these changes led to lower transverse stability (lower restoring moment for a given angle of roll) with the consequence of larger roll motion. The increase in roll motion and its effect on cargo and human performance lead to the development several control devices that aimed at reducing and controlling roll motion. The control devices most commonly used today are fin stabilizers, rudder, anti-roll tanks, and gyrostabilizers. The use of different types of actuators for control of ship roll motion has been amply demonstrated for over 100 years. Performance, however, can still fall short of expectations because of difficulties associated with control system design, which have proven to be far from trivial due to fundamental performance limitations and large variations of the spectral characteristics of wave-induced roll motion. This short article provides an overview of the fundamentals of control design for ship roll motion reduction. The overview is limited to the most common control devices
Motion sensing on vessels operating in sea ice: A local monitoring system for transit and stationkeeping operations under the influence of sea ice
No full textThe diminishing sea ice in the Arctic and Antarctic could lead to a higher number of ship operations in these areas, such as cargo transit, resource exploration, fishing, and tourism. However, despite the sea ice reduction, sea ice remains the predominant risk during ship operations. Due to the remoteness and fragility of the polar regions, accidents are difficult to handle and could have devastating effects on the local ecosystem. Therefore, continuous assessing of the prevailing ice conditions is essential to operating vessels. Individual sensor systems, called ice monitoring systems, provide the necessary information for the ice condition assessment, and by giving early warnings, these systems reduce the risk of accidents.
A reliable ice observer system employs several technologies for ice monitoring such as optical cameras, radar systems, drift buoys, and hull strain measurements. Each additional technology increases the chance of early detection of dangerous ice conditions, and additionally adds redundancies to the overall system.
The aim of this thesis was to present and validate an applicable ice monitoring system that can increase the safety for vessels operating in polar regions. This thesis presents a series of studies, presented as a collection of journal papers, that lead to an on-board motion sensing based ice monitoring system for ships, which bases on distributed measurements of ice-induced vibrations in the ship’s hull. The results of this thesis are based on field data collected during four Arctic cruises performed between 2015 and 2017.
An initial study established a relationship between the prevailing ice conditions and ice-induced vibrations in the ship’s hull. A detailed frequency analysis of ice-induced vibrations concluded that accelerometers in the hull can provide information about the acting ice breaking mechanism, ice conditions around the vessel, and the location of ship-ice interaction along the hull. Two further studies established and validated the application of hull accelerometers as ice monitoring system. A first application study suggested to monitor ice conditions in real time with the help of statistical signal processing and change detection. It was found that the suggested ice monitoring system provides robust real-time information about the local ice conditions and operates independently of weather conditions. A second application study utilised several sensors along the hull, and methods from extreme value statistics, to find a relationship between the ice drift direction and statistical properties of the recorded signals. The proposed methods in this thesis would allow for an early detection of ice drift changes, which is essential for station-keeping operations in sea ice.
The overall research presented in this thesis conclude that motion sensing on vessels operating in sea ice provides a fast and reliable local ice monitoring system for both transit and station-keeping operations. It is further suggested, that motion sensing in the hull of a vessel also provides information about the current sea-state in open water. However, this possible application is outside the scope of this thesis.
Besides the main studies, the thesis additionally offers a contribution in form of a model for distributed motion sensing on ships.digital fulltext not avialabl
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
- …
