10,291 research outputs found
Attitude analysis in Flatland: The plane truth
Many results in attitude analysis are still meaningful when the attitude is restricted to rotations about a single axis. Such a picture corresponds to attitude analysis in the Euclidean plane. The present report formalizes the representation of attitude in the plane and applies it to some well-known problems. In particular, we study the connection of the 'additive' and 'multiplicative' formulations of the differential corrector for the quaternion in its two-dimensional setting
New quests for better attitudes
During the past few years considerable insight was gained into the QUEST algorithm both as a maximum likelihood estimator and as a Kalman filter/smoother for systems devoid of dynamical noise. The new algorithms and software are described and analytical comparisons are made with the more conventional attitude Kalman filter. It is also described how they may be accommodated to noisy dynamical systems
General discussion : central bank communication and policy effectiveness
Greenspan, Alan ; Monetary policy ; Banks and banking, Central
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Presynaptic facilitation at the crayfish neuromuscular junction: Role of calcium-activated potassium conductance
Membrane potential was recorded intracellularly near presynaptic
terminals of the excitor axon of the crayfish opener neuromuscular junction (NMJ),
while transmitter release was recorded postsynaptically. This study focused on the
effects of a presynaptic calcium-activated potassium conductance, gK(Ca), on the
transmitter release evoked by single and paired depolarizing current pulses.
Blocking gK(Ca) by adding tetraethylammonium ion (TEA; 5-20 mM) to a solution
containing tetrodotoxin and aminopyridines caused the relation between presynaptic
potential and transmitter release to steepen and shift to less depolarized
potentials. When two depolarizing current pulses were applied at 20-ms intervals
with gK(Ca) not blocked, the presynaptic voltage change to the second (test) pulse was
inversely related to the amplitude of the first (conditioning) pulse. This effect of the
conditioning prepulse on the response to the test pulse was eliminated by 20 mM
TEA and by solutions containing 0 mM Ca2+/1 mM EGTA, suggesting that the
reduction in the amplitude of the test pulse was due to activation of gK(Ca) by calcium
remaining from the conditioning pulse. In the absence of TEA, facilitation of
transmitter release evoked by a test pulse increased as the conditioning pulse grew
from -40 to -20 mV, but then decreased with further increase in the conditioning
depolarization. A similar nonmonotonic relationship between facilitation and the
amplitude of the conditioning depolarization was reported in previous studies using
extracellular recording, and interpreted as supporting an additional voltagedependent
step in the activation of transmitter release. We suggest that this result
was due instead to activation of a gK(Ca) by the conditioning depolarization, since
facilitation of transmitter release increased monotonically with the amplitude of the
conditioning depolarization, and the early time course of the decay of facilitation
was prolonged when gK(Ca) was blocked. The different time courses for decay of the
presynaptic potential (20 ms) and facilitation (> 50 ms) suggest either that residual free calcium does not account for facilitation at the crayfish NMJ or that the
transmitter release mechanism has a markedly higher affinity or stoichiometry for
internal free calcium than does g K(Ca). Finally, our data suggest that the calcium
channels responsible for transmitter release at the crayfish NMJ are not of the L, N,
or T type.This work was partially supported by NIAAA grant AA0776 to G. D. Bittner.Neuroscienc
Attitude sensor alignment calibration for the solar maximum mission
An earlier heuristic study of the fine attitude sensors for the Solar Maximum Mission (SMM) revealed a temperature dependence of the alignment about the yaw axis of the pair of fixed-head star trackers relative to the fine pointing Sun sensor. Here, new sensor alignment algorithms which better quantify the dependence of the alignments on the temperature are developed and applied to the SMM data. Comparison with the results from the previous study reveals the limitations of the heuristic approach. In addition, some of the basic assumptions made in the prelaunch analysis of the alignments of the SMM are examined. The results of this work have important consequences for future missions with stringent attitude requirements and where misalignment variations due to variations in the temperature will be significant
Feedback Synthesis for Controllable Underactuated Systems using Sequential Second Order Actions
This paper derives nonlinear feedback control synthesis for general control
affine systems using second-order actions---the needle variations of optimal
control---as the basis for choosing each control response to the current state.
A second result of the paper is that the method provably exploits the nonlinear
controllability of a system by virtue of an explicit dependence of the
second-order needle variation on the Lie bracket between vector fields. As a
result, each control decision necessarily decreases the objective when the
system is nonlinearly controllable using first-order Lie brackets. Simulation
results using a differential drive cart, an underactuated kinematic vehicle in
three dimensions, and an underactuated dynamic model of an underwater vehicle
demonstrate that the method finds control solutions when the first-order
analysis is singular. Moreover, the simulated examples demonstrate superior
convergence when compared to synthesis based on first-order needle variations.
Lastly, the underactuated dynamic underwater vehicle model demonstrates the
convergence even in the presence of a velocity field.Comment: 9 page
Effects of deposit-feeding bivalve (Macomona liliana) density on intertidal sediment stability
Effects of macrofaunal feeding and bioturbation on intertidal sediment stability (u*crit) were investigated by manipulating density (0-3 x ambient) of the facultative deposit-feeding wedge shell (Macomona liliana) on the Tuapiro sandflat in Tauranga Harbour, New Zealand. Sediment stability increased up to 200% with decreasing M. liliana density and this was correlated with greater sediment microalgal biomass and mucilage content. The change in stability occurred despite homogeneity of grain size amongst experimental treatments, highlighting the importance of macrofaunal-microbial relationships in determining estuarine sediment erodibility
Bored pile design in stiff clay II:Mechanisms and uncertainty
The soil mechanics related to pile design in clay has been the subject of substantial engineering research. In a companion paper, various codes of practice were reviewed showing the effect on pile capacity of the different global factors of safety that emerge from the various partial factor combinations for the ultimate limit state. Factors of safety are generally specified based on the opinions of experts. In this paper an assessment will be made of various objective procedures that can be used to reduce uncertainty in the design process, especially regarding the adoption of a pile resistance model and the selection of a soil strength profile as part of a ultimate limit state check, and the estimation of pile head settlement in the context of a serviceability limit state check. It is shown that both total stress and effective stress calculation methods are applicable in London Clay. Estimates of settlement using a non-linear soil stress–strain relationship are made and compared with published data. It is shown that the compression of the concrete dominates the settlement of long piles. Given the low settlements observed, recommendations are made for a reduction in standard factors of safety for bored pile design in stiff clays. </jats:p
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