Development and experimentation of LQR/APF guidance and control for autonomous proximity maneuvers of multiple spacecraft

http://dx.doi.org/10.1016/j.actaastro.2010.08.012This work introduces a novel control algorithm for close proximity multiple spacecraft autonomous maneuvers, based on hybrid linear quadratic regulator/artificial potential function (LQR/APF), for applications including autonomous docking, on-orbit assembly and spacecraft servicing. Both theoretical developments and experimental validation of the proposed approach are presented. Fuel consumption is sub-optimized in real-time through re-computation of the LQR at each sample time, while performing collision avoidance through the APF and a high level decisional logic. The underlying LQR/APF controller is integrated with a customized wall-following technique and a decision logic, overcoming problems such as local minima. The algorithm is experimentally tested on a four spacecraft simulators test bed at the Spacecraft Robotics Laboratory of the NAval Postgraduate School. The metrics to evaluate the control algorithm are: autonomy of the system in making decisions, successful completion of the maneuver, required time, and propellant consumption

Material processing with pulsed radially and azimuthally polarized laser radiation

We report on the generation of radially and azimuthally polarized Q-switched laser radiation and its application in material processing. The power levels were sufficiently high to study micro-hole drilling in different metals. Depending on the optical properties of the metal, either radial or azimuthal polarization shows the best efficiency and the effect is attributed to waveguiding. For steel, a comparison to linearly or circularly polarized laser radiation indicates that the doughnut-shaped beam with azimuthal polarization is the most energy-efficient in producing holes of the same diameter and dept

Development and experimentation of LQR/APF guidance and control for autonomous proximity maneuvers of multiple spacecraft

http://dx.doi.org/10.1016/j.actaastro.2010.08.012This work introduces a novel control algorithm for close proximity multiple spacecraft autonomous maneuvers, based on hybrid linear quadratic regulator/artificial potential function (LQR/APF), for applications including autonomous docking, on-orbit assembly and spacecraft servicing. Both theoretical developments and experimental validation of the proposed approach are presented. Fuel consumption is sub-optimized in real-time through re-computation of the LQR at each sample time, while performing collision avoidance through the APF and a high level decisional logic. The underlying LQR/APF controller is integrated with a customized wall-following technique and a decision logic, overcoming problems such as local minima. The algorithm is experimentally tested on a four spacecraft simulators test bed at the Spacecraft Robotics Laboratory of the NAval Postgraduate School. The metrics to evaluate the control algorithm are: autonomy of the system in making decisions, successful completion of the maneuver, required time, and propellant consumption

Separaxion anxiety in pediatric migraine without aura: A pilot study

Background: Separation anxiety (SA) can be defined as the fear reaction and protest manifested by children when the main caregivers move away from him/her or in front of unfamiliar person. SA near eight months may be considered as an important and normal phase of the correct and typical social neurodevelopment. Aims of the present pilot study is assessing the prevalence of separation anxiety in a population of patients with migraine without aura (MwA). Materials and methods: 119 children (69 males) suffering from MwA (mean age 1.78 ± 7:59) were consecutively recruited. The control population consisted of 231 (114 males) healthy subjects similar for age (7.64 ± 1:34; p = 0.768) and gender (p=0.987) The Screen for child anxiety related emotional disorders (SCARED) test was used to assess the prevalence of separation anxiety among MwA children. Results: Healthy individuals are on average less affected (mean 4.72 ± 0:32) of the Separation Anxiety Disorder respect of MwA children (mean 6.83 ± 0.97; p < 0.001) (Figure 1). Conclusion. MwA presents many psychiatric comorbidities and among ones separation anxiety may be considered in the clinical and therapeutic management of pediatric primary headache

The 2003 eclipse of EE Cep is coming. A review of past eclipses

EE Cep is an eclipsing binary with a period of 5.6 years. The next eclipse will occur soon, in May-June 2003, and all available past eclipses were collected and briefly analysed. EE Cep shows very large changes of the shape and the depth of minima during different eclipses, however it is possible to single out some persistent features. The analysis suggests that the eclipsing body should be a long object surrounded by an extended semi-transparent envelope. As an explanation, a model of a precessing optically thick disc, inclined to the plane of the binary orbit, is invoked. The changes of its spatial orientation, which is defined by the inclination of the disc and the tilt, induced most probably by precession of the disc spin axis with a period of about 50 years, produce strange photometric behaviour of this star. The H_alpha emission, and possibly the NaI absorptions, show significant changes during several months outside of the eclipse phase.Comment: 7 pages, 7 figures, LaTeX2e, accepted by A&

Effect of Lupinus albus as protein supplement on yield, constituents, clotting properties and fatty acid composition in ewes' milk

The effect of feeding lupin seeds (Lupinus albus L.) as an alternative protein source in ewe diets was investigated. Two groups of 18 Sarda ewes were fed two different isonitrogenous diets: with lupin (L) seed, given after 12 h soaking, or soybean meal (SBM) as the main protein source. DMI, variations of body weight and milk production were unaffected by the treatment. Although not statistically significant, in the group fed L diet the production of milk fat and protein was higher. Clotting properties of milk were similar for the two treatments, probably due to the small differences in the milk protein contents. The fatty acid profile of milk was affected by treatment with a larger content of short (14.19 wt% versus 12.26 wt%)- and medium (49.37 wt% versus 47.76 wt%)-chain fatty acids in milk from ewes fed the L diet. CLA content was unaffected by treatment. Triglyceride content of fat from the two diets reflects the milk fatty acid composition. Indeed, milk from L diet showed a higher level of medium-chain triglycerides, which are of particular interest to consumers with concerns over health and heart disease. The inclusion of lupin seed in the diet of lactation ewes can be a means of achieving a more desirable triglyceride profile in milk fat. Milk with enhanced nutritive quality may promote wider market penetration of sheep dairy products

A possible new phase of antagonistic nematogens in a disorienting field

A simple model is proposed for nematogenic molecules that favor perpendicular orientations as well as parallel ones. (Charged rods, for example, show this antagonistic tendency.) When a small disorienting field is applied along $z$, a low density phase $N_-$ of nematic order parameter $S_z<0$ coexists with a dense biaxial nematic $N_b$. (At zero field, $N_-$ becomes isotropic and $N_b$ uniaxial.) But at stronger fields, a new phase $N_{+4}$, invariant under $\pi/2$ rotations around the field axis, appears in between $N_-$ and $N_b$. Prospects for finding the $N_{+4}$ phase experimentally are briefly discussed.Comment: 4 pages, 2 figures. Accepted for publication in PR

Carbon and nitrogen abundances of individual stars in the Sculptor dwarf spheroidal galaxy

We present [C/Fe] and [N/Fe] abundance ratios and CH({\lambda}4300) and S({\lambda}3883) index measurements for 94 red giant branch (RGB) stars in the Sculptor dwarf spheroidal galaxy from VLT/VIMOS MOS observations at a resolving power R= 1150 at 4020 {\AA}. This is the first time that [N/Fe] abundances are derived for a large number of stars in a dwarf spheroidal. We found a trend for the [C/Fe] abundance to decrease with increasing luminosity on the RGB across the whole metallicity range, a phenomenon observed in both field and globular cluster giants, which can be interpreted in the framework of evolutionary mixing of partially processed CNO material. Both our measurements of [C/Fe] and [N/Fe] are in good agreement with the theoretical predictions for stars at similar luminosity and metallicity. We detected a dispersion in the carbon abundance at a given [Fe/H], which cannot be ascribed to measurement uncertainties alone. We interpret this observational evidence as the result of the contribution of different nucleosynthesis sources over time to a not well-mixed interstellar medium. We report the discovery of two new carbon-enhanced, metal-poor stars. These are likely the result of pollution from material enriched by asymptotic giant branch stars, as indicated by our estimates of [Ba/Fe]> +1. We also attempted a search for dissolved globular clusters in the field of the galaxy by looking for the distinctive C-N pattern of second population globular clusters stars in a previously detected, very metal-poor, chemodynamical substructure. We do not detect chemical anomalies among this group of stars. However, small number statistics and limited spatial coverage do not allow us to exclude the hypotheses that this substructure forms part of a tidally shredded globular cluster.Comment: 18 pages, 14 figures, 3 tables. Accepted to A&

Quasistationary binary inspiral. I. Einstein equations for the two Killing vector spacetime

The geometry of two infinitely long lines of mass moving in a fixed circular orbit is considered as a toy model for the inspiral of a binary system of compact objects due to gravitational radiation. The two Killing fields in the toy model are used, according to a formalism introduced by Geroch, to describe the geometry entirely in terms of a set of tensor fields on the two-manifold of Killing vector orbits. Geroch's derivation of the Einstein equations in this formalism is streamlined and generalized. The explicit Einstein equations for the toy model spacetime are derived in terms of the degrees of freedom which remain after a particular choice of gauge.Comment: 37 pages, REVTeX, one PostScript Figure included with epsfig; minor formatting changes and copyright notice added for journal publicatio