Contraction analysis of switched Filippov systems via regularization

We study incremental stability and convergence of switched (bimodal) Filippov systems via contraction analysis. In particular, by using results on regularization of switched dynamical systems, we derive sufficient conditions for convergence of any two trajectories of the Filippov system between each other within some region of interest. We then apply these conditions to the study of different classes of Filippov systems including piecewise smooth (PWS) systems, piecewise affine (PWA) systems and relay feedback systems. We show that contrary to previous approaches, our conditions allow the system to be studied in metrics other than the Euclidean norm. The theoretical results are illustrated by numerical simulations on a set of representative examples that confirm their effectiveness and ease of application.Comment: Preprint submitted to Automatic

Isolated large amplitude periodic motions of towed rigid wheels

This study investigates a low degree-of-freedom (DoF) mechanical model of shimmying wheels. The model is studied using bifurcation theory and numerical continuation. Self-excited vibrations, that is, stable and unstable periodic motions of the wheel, are detected with the help of Hopf bifurcation calculations. These oscillations are then followed over a large parameter range for different damping values by means of the software package AUTO97. For certain parameter regions, the branches representing large amplitude stable and unstable periodic motions become isolated following an isola birth. These regions are extremely dangerous from an engineering view-point if they are not identified and avoided at the design stage.Comment: Appeared online in Nonlinear Dynamics Thursday, April 26, 200

Impact of Water Recovery from Wastes on the Lunar Surface Mission Water Balance

Future extended lunar surface missions will require extensive recovery of resources to reduce mission costs and enable self-sufficiency. Water is of particular importance due to its potential use for human consumption and hygiene, general cleaning, clothes washing, radiation shielding, cooling for extravehicular activity suits, and oxygen and hydrogen production. Various water sources are inherently present or are generated in lunar surface missions, and subject to recovery. They include: initial water stores, water contained in food, human and other solid wastes, wastewaters and associated brines, ISRU water, and scavenging from residual propellant in landers. This paper presents the results of an analysis of the contribution of water recovery from life support wastes on the overall water balance for lunar surface missions. Water in human wastes, metabolic activity and survival needs are well characterized and dependable figures are available. A detailed life support waste model was developed that summarizes the composition of life support wastes and their water content. Waste processing technologies were reviewed for their potential to recover that water. The recoverable water in waste is a significant contribution to the overall water balance. The value of this contribution is discussed in the context of the other major sources and loses of water. Combined with other analyses these results provide guidance for research and technology development and down-selection

Early visual contributions to reading

Reading requires integrating visual and linguistic processes, so it is perhaps surprising that models of visual word recognition focus almost entirely on language, to the exclusion of vision. Neurological models of reading assume that visual information proceeds serially from the retina through the early visual cortices, where a hierarchy of increasingly complex feature detectors transform the sensory-bound retinotopic code into progressively more abstract forms, eventually reaching reading-specialised populations that encode orthographic units. These orthographic detectors are the input pathway to the wider language system. This notion of a serial staged hierarchy culminating in abstract detectors, however, is likely overly simplistic. Evidence suggests that the occipitotemporal system is better understood as a highly recurrent network. Classical hierarchical accounts and interactive processing models make contrasting predictions about how early visual areas contribute to reading. To test these predictions, I retinotopically mapped occipital visual areas and then measured (fMRI) their neural response to different reading tasks. I found that reading strongly engaged areas V1-V3 bilaterally, both in the central (stimulated) regions and in regions coding the periphery, suggesting both bottom-up and top-down influences in early visual cortices. Within the central regions of V1-V3, activity was significantly stronger for low frequency than high frequency words, again suggesting that non-visual factors such as lexical frequency influence processing in the earliest visual areas. Subsequent analyses revealed that ventral (V4, VO-1, VO-2) and dorsal (V3a, V3b, V7) regions were both active during reading, with no evidence of any difference in the strength of activation between them. Lastly, I found that a seemingly incidental property of the experimental paradigm (stimulus presentation rate) dramatically affected V1-V3 activity. Together these results contradict the notion that the early visual stages of reading are tightly sensory-bound and require reading-specific neuronal representations. Rather, these findings suggest that reading is an interactive process, even in the earliest visual cortices

Balancing of the skateboard with reflex delay

A simple mechanical model of the skateboard-skater system is constructed in which a PD controller with time delay is implemented. Equations of motion are derived with the help of Appell-Gibbs method and are linearized around straight uniform motion. The linear stability analysis is carried out analytically using the D-subdivision method. Stability charts are presented for realistic system parameters and the critical time delay is calculated. It is also shown how the control gains have to be varied with respect to the speed of the skateboard in order to stabilize uniform motion