1,012 research outputs found
Influential stimuli characteristics on SNR in SSVEP-based interfaces: Thesis report: researching different aspects that influence the SNR in SSVEP-based interfaces
No full textDifferent paradigms can be used to evoke brainwaves. These brainwaves can be interpreted as commands that can be used to control different applications. These frameworks that interpret the brainwaves are called brain-computer interfaces. Steady-state visually evoked potentials is one of these paradigms that uses external stimuli flickering at fixed frequencies to evoke brainwaves. This paradigm is fast to issue, is reliable, and needs no training time of the user. However, to be able to distinguish multiple commands it is important to distinguish between multiple commands reliably. The fundamental metric that determines the signal quality is the signal-to-noise ratio. The signal-to-noise ratio is measured in decibels and is the ratio in power between the signal that is evoked around the stimulus frequency and the power of some baseline, referred to as noise. To extract the power the discrete Fourier transform is calculated from the measured brainwaves. The brainwaves are measured in millivolts over time. These brainwaves can be recorded using implants in the head named intracortical or external, such as electroencephalography. External methods such as electroencephalography are much safer for the user. Important to recognize is that one of the contributing factors to signal-to-noise ratio are the characteristics of the external stimuli displayed. Research is still trying to figure out the exact relationship between stimuli characteristics and signal-to-noise ratio. However, the experiments of previous research lack the context of the gaze of the subjects to explain the electroencephalography recordings. In this research, this is attempted to be solved using eye tracking. Furthermore, there is a research gap in thethe scientific field surrounding signal-to-noise ratio and stimuli characteristics as the effect of surroundingstimuli on the measured signal-to-noise ratio of the target stimulus has never been investigated.This research attempted to solve these problems by performing 2 experiments with 6 participants. One experiment shows a single stimulus across various shapes (triangles, squares, and circles), colors (red, green, and white), frequencies (9, 13, 19, and 25Hz), and sizes (10.000, 20.000, 30.000 pixels). The other experiment simulates the natural environment of the external stimuli across the same frequencies, colors, and shapes. The natural environment of a single stimulus is actually surrounding stimuli at different frequencies, as applications often require the ability to distinguish between multiple different commands. Thus, to state the main research question: "What is the relationship between the stimuli characteristics and the measured SNR?".This question is answered by dissecting the effect that color, shape, size, frequency, and surrounding stimuli have on the signal-to-noise ratio. Dr.ir. Y.B. Eisma was the supervisor of S.T. van Vliet. Using his equipment the brainwaves were recorded at the UMC Amsterdam. https://github.com/SjoerdTimovanVliet/SSVEP_interface_thesisMechanical Engineering | Vehicle Engineering | Cognitive Robotic
Processes controlling the geochemical composition of the South China Sea sediments during the last climatic cycle
No full textAuthor Posting. © Elsevier B.V., 2008. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Chemical Geology 257 (2008): 240-246, doi:10.1016/j.chemgeo.2008.10.002.Sediments of the upper 28.2 meters of Ocean Drilling Program (ODP) Site 1145 from
the northern South China Sea (SCS) were analyzed for their geochemical composition. Most
of the major and trace elements exhibit significant fluctuations at glacial-interglacial scales,
implying a close relation with regional and global climate change. Al-normalized elemental
ratios can be subdivided into three principal components (PC). PC1 (e.g., Ca/Al, Ba/Al, Sr/Al)
displays significant glacial-interglacial variation and is related to paleoproductivity in the
northern SCS. PC2 (e.g., K/Al, Mg/Al, Rb/Al) is associated with the degree of chemical
weathering in the source regions and shows little glacial-interglacial variation. PC3 (e.g.,
Ti/Al, Zr/Al) reflects the relative contribution of coarse- and fine-grained materials in the
terrigenous components of the SCS sediments, likely associated with changes in sea level and
monsoon-induced fluvial input. Spectral analyses indicate that paleoproductivity (i.e., Ba/Al)
in the South China Sea lags Hulu/Sanbao speleothem δ18O record (a indicator of annual
average meteoric precipitation) by 102° and Indian summer monsoon (multi-proxy stack) by
23° at the precession band, indicating a close relationship with the Indian summer monsoon.
However, the chemical weathering degree in the source area (PC2) is not sensitive to
monsoon-related changes at the precession band during the last climatic cycle.This study was supported by the NSFC to Y.B. Sun and the US NSF to D.W. Oppo (OCE 0502960) and S.C. Clemens (OCE 0352215)
Optimizing deep reinforcement learning policies for deteriorating systems considering ordered action structuring and value of information
No full textInspection and maintenance (I&M) optimization entails many sources of computational complexity, among others, due to high-dimensional decision and state variables in multi-component systems, long planning horizons, stochasticity of objectives and constraints, and inherent uncertainties in measurements and models. This paper studies how the above can be addressed within the context of constrained Partially Observable Markov Decision Processes (POMDPs) and Deep Reinforcement Learning (DRL) in a unified fashion. Special emphasis is paid on how ordered action structuring of I&M actions can be exploited to decompose the respective policy parametrizations in actor-critic DRL schemes, resulting into fully decoupled maintenance and inspection actors. It is shown that the Value of Information (VoI) is naturally utilized in such POMDP control frameworks, as directly associated with the DRL advantage functions that emerge in the gradient computations of the inspection policy parameters. Overall, the presented approach, following the natural flow of engineering decisions, results in new architectural configurations for policy networks, facilitating more efficient training, while alleviating further the dimensionality burdens related to combinatorial definitions of I&M actions. The efficiency of the methodology is demonstrated in numerical experiments of a structural system subject to corrosion, where the optimization problem is formulated to concurrently account for state and model uncertainties as well as long-term probability of failure exceedance constraints. Results showcase that the obtained DRL policies considerably outperform standard decision rules.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Structural Design & Mechanic
Model induction from data: Towards the next generation of computational engines in hydraulics and hydrology
No full textCivil Engineering and Geoscience
Long-term morphological modeling for Feiyun estuary with ESTMORF
No full textThe Feiyun River, with a length of 203 km and a catchment area of 3252 km2, is one of eight rivers in Zheijiang Province (see fig.1-1), China. It flows into the East China Sea in Shangwang of Ruian City. The source of Feiyun River is at Xialing located in the border between Zheijiang and Fujian Province. The tidal influence can reach up the river as far as Tanjiao located 59 km from the river mouth in the dry season during spring tide. The downstream river reach of Tanjiao is called the estuary reach mainly controlled by tidal flow, and its upstream is called river reach controlled by the basin runoff. The Feiyun Estuary connects the sea to Ruian harbour. There is an old and a new harbour zone near the Feiyun first bridge in Ruian City. The old harbour is only suitable for 300T to 500T ships. A quay berth for 1000T ship is in the new part. With the development of the economics and foreign trade of Ruian City, the harbour is developing rapidly. Therefore, it is necessary for local government to carry out the channel improvement of the Feiyun estuary. A lot of human interference's are going on / planned in the Feiyun estuary. In the upstream reach of the Feiyun River, the large scale Sanxi reservoir has been constructed and began to store water. Further the diversion from the Zhaoshadu reservoir will also be carried out. A bend cut-off project is also in schedule in order to improve the flood defence and create land for Ruian City. It is no doubt that all these human activities will have important effect on the long-term morphological development of the Feiyun estuary, especially for the navigation channel of the downstream section of the Feiyun estuary. Within the framework of Delft Cluster project Ecomorphology of Estuaries and Coasts, the longterm morphological development of the Feiyun estuary has been carried out with the ESTMORF model, which belongs to the hybrid class of the models. The impact of the bend cut-off in the Feiyun estuary, the construction of the Sanxi reservoir and the diversion project, to the long-term morphological development is studied.Hydraulic EngineeringCivil Engineering and Geoscience
Visual Attention in Human−Machine Interaction
No full textHumans are incapable of attending to everything at the same time. The serial nature of focused attention limits the information intake capacity of the perceptual system. This thesis deals with the measurement and modelling of visual attention distribution. It is examined whether measures of visual attention are predictive of task performance. The first chapter introduces the main topic of this thesis: the complex nature of modern technological systems, which feature many information sources that have to be monitored. Many psychological constructs have been proposed in the human factors literature that have alleged criterion validity for task performance. Here, task performance is regarded as the human’s ability to e.g., take over control of an automated system in potential critical situations. Contrary to the speculative nature of some of the Human- Factors constructs, this thesis sets out to capture performance in terms of objective measures of visual attention. Wickens’s (2008) Salience, Effort, Expectancy, Value (SEEV) model is introduced and discussed. This model is utilized for interpreting the eye-tracking results. Finally, a rationale for the topics in the thesis is provided. Chapters 2 through 4 of this thesis discuss and elaborate on Senders’s (1983) research in detail, by means of replication research and an extensive tutorial on his mathematical models. These chapters provide an empirical underpinning and conceptual understanding of the concept of visual attention. Chapters 5 through 8 discuss visual attention in light of Air Traffic Control (ATC) and automated driving, and are regarded as suitable cases for attention distribution measurement and task performance prediction. Chapter 9 investigates task performance and visual attention in a psychometric task: Inspection Time, which provides a good testbed for operationalizing the effect of attention on task performance. Chapter 10 concludes with a discussion on the topics in this thesis...Human-Robot Interactio
Horizontal shear flows over a streamwise varying bathymetry
No full textThe Eastern Scheldt storm surge barrier is an icon of Dutch hydraulic engineering. Downstream of the barrier, large local erosion pits (scour holes) have formed adjacent to the applied bed protection after its construction. It was expected during the design phase that these would develop, but both the magnitude of the scour hole depth as well as the present continuation of the scour hole development were not foreseen. In this thesis, the fundamental fluid mechanical behaviour of the flow around the scour holes was studied through a combination of field data analysis, mathematical modelling and laboratory experiments. Field data showed that the flow near the barrier is characterized by large transverse differences in streamwise velocity, and the flow is thus classified as a horizontal shear flow. As such a flow develops over a locally varying bathymetry in streamwise direction, it showcases non-intuitive behaviour, with potentially large consequences for the ongoing scour development. As a horizontal shear flow develops over a streamwise oriented increase in flow depth, its streamlines either converge or diverge in the horizontal plane. Associated with either horizontal convergence or divergence is the absence or presence, respectively, of vertical flow separation. In case of horizontal convergence and suppression of vertical flow separation, the bed shear stress is shown to be significantly higher compared to a flow that horizontally diverges and vertically separates. The rate of horizontal convergence was shown to be dependent on the increase in flow depth; thus, a positive feedback mechanism is revealed where the presence of a local increase in flow depth sustains or even enhances further development of such a feature. It was demonstrated through mathematical modelling that in case of horizontal convergence and vertical attachment, a streamwise oriented increase in flow depth led to an intensification of the turbulence structures. The findings from this study were used to explain the observed ongoing growth of the scour holes near the Eastern Scheldt storm surge barrier. Besides, it was hypothesized how the phenomena as revealed in this thesis would apply to similar configurations as the Eastern Scheldt storm surge barrier. The results from this thesis may form part of the knowledge base from which design guidelines or (numerical) design tools for protection against scour around hydraulic structures are developed.Environmental Fluid Mechanic
Next-generation wind turbine tower modeling: Uncertainty quantification
No full textThe natural frequency of wind turbine towers can be more than 10% off from the designed frequency. This frequency is important for control because it sets bandwidth limits to some of the control loops. Improving the model will result in better control and load reduction that will in turn contribute to the decrease of the cost of wind energy. The main focus of this project is on uncertainty quantification. Finding more accurate uncertainty quantification techniques will help determining the accuracy of models. Currently, the accuracy of the models is described based on asymptotic statistics theory. However, asymptotic uncertainty descriptions exhibit some problems. Variance results are not reliable for small numbers of measurement samples. Moreover, the true values of the parameters are present in the asymptotic variance expressions. As a result, the uncertainty of the model is often approximately quantified or unknown. When the uncertainty of the model is not known or approximately quantified the controllers have to be designed with conservative margins. Therefore, obtaining more reliable uncertainty descriptions will enable the use of less conservative margins, and thus more aggressive controllers. For this purpose, the bootstrap technique for accuracy quantification of model estimates is proposed. Additionally, Operational Modal Analysis (OMA) is used to obtain an estimate of the natural frequency of the tower. With OMA a model of a system is identified based on the output data collected when the system is in operating conditions. The use of OMA eliminates the need to artificially excite the wind turbine tower by using artificial exciters. Another advantage of OMA over system identification approaches is that the input signal to the system is not measured and only the output vibration response of the tower is measured. In this project, the system identification method optimized Predictor Based System Identification (PBSIDopt) was adapted to work with output only data. The identification procedure was tested on a simulation example and the results showed that the proposed identification based on ambient excitation was effective. Asymptotic variance expressions for the uncertainty of the natural frequencies and damping ratios have been derived for the OMA-PBSIDopt. The uncertainty of the natural frequency and damping ratio of the identified system model has been quantified using both the asymptotic statistics and bootstrap techniques and a comparison of the results is made. It is shown that the bootstrap method outperforms the asymptotic variance approach in the sense that the bootstrap estimates of the accuracy are less sensitive to the number of measurements samples. In addition to that, unlike the asymptotic variance technique the bootstrap method is straightforward to apply.Systems and ControlDelft Center for Systems and ControlMechanical, Maritime and Materials Engineerin
Visual sampling processes revisited: replicating and extending senders (1983) using modern eye-tracking equipment
No full textIn pioneering work, Senders (1983) tasked five participants to watch a bank of six dials, and found that glance rates and times glanced at dials increase linearly as a function of the frequency bandwidth of the dial's pointer. Senders did not record the angle of the pointers synchronously with eye movements, and so could not assess participants’ visual sampling behavior in regard to the pointer state. Because the study of Senders has been influential but never repeated, we replicated and extended it by assessing the relationship between visual sampling and pointer state, using modern eye-tracking equipment. Eye tracking was performed with 86 participants who watched seven 90-second videos, each video showing six dials with moving pointers. Participants had to press the spacebar when any of the six pointers crossed a threshold. Our results showed a close resemblance to Senders’ original results. Additionally, we found that participants did not behave in accordance with a periodic sampling model, but rather were conditional samplers, in that the probability of looking at a dial was contingent on pointer angle and velocity. Finally, we found that participants sampled more in agreement with Nyquist sampling when the high bandwidth dials were placed in the middle of the bank rather than at its outer edges. We observed results consistent with the saliency, effort, expectancy, and value model and conclude that human sampling of multidegree of freedom systems should not only be modeled in terms of bandwidth but also in terms of saliency and effort.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Control & SimulationIntelligent VehiclesBiomechatronics & Human-Machine Contro
Modelling of Grain Sorting Mechanisms in the Nearshore Area for Natural and Nourished Beaches
No full textIn this study a modelling approach was investigated that takes into account the effect of short wave grouping on long wave motions, calculating sediment transport and morphology with multiple fractions and a layered bed stratigraphy. After extensive testing and validating this model using data collected in a wave flume in Hannover this model was used to simulate morphological development at nourished beaches. Based on the model results, an assessment on grain sorting effects during the development of nourished beaches was made, showing the models capabilities in functioning as a tool to predict sorting processes for natural and nourished beaches in a schematized environment.Coastal EngineeringHydraulic EngineeringCivil Engineering and Geoscience
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