1,148 research outputs found
Solar vane actuator Patent
Design and characteristics of device for sensing solar radiation and providing spacecraft attitude control to maintain direction with respect to incident radiatio
Attitude control for spacecraft Patent
Attitude control system for spacecraft based on conversion of incident solar radiation on movable control surfaces into mechanical torque
Optimizing Attitude Control Systems
This paper discusses a practical approach to attitude control system mechanization. Previous efforts reported in the literature have either resulted in systems too complex to mechanize or have not considered the problem in enough of its aspects to make the work meaningful. The classical control optimization techniques are briefly summarized and a critique of these methods is given. The solutions obtained with the classical techniques are either open loop, which is unsatisfactory from an attitude control standpoint, or they are closed loop. These closed loop solutions in general may require measuring all of the system variables, which may not be possible, or they may be far too complex to mechanize. In the proposed approach, called specific optimal control, sensor and actuator characteristics are given and the form of the controller is chosen. Controller parameters are then chosen so as to minimize some performance index. Three analytical methods being developed to perform this optimization are hill climbing, two point boundary value problem formulation, and differential approximation. Each of these methods are discussed. Numerical examples showing the application of these techniques are given in a reference
Large zero-field cooled exchange-bias in bulk Mn2PtGa
We report a large exchange-bias (EB) effect after zero-field cooling the new
tetragonal Heusler compound Mn2PtGa from the paramagnetic state. The
first-principle calculation and the magnetic measurements reveal that Mn2PtGa
orders ferrimagnetically with some ferromagnetic (FM) inclusions. We show that
ferrimagnetic (FI) ordering is essential to isothermally induce the exchange
anisotropy needed for the zero-field cooled (ZFC) EB during the virgin
magnetization process. The complex magnetic behavior at low temperatures is
characterized by the coexistence of a field induced irreversible magnetic
behavior and a spin-glass-like phase. The field induced irreversibility
originates from an unusual first-order FI to antiferromagnetic transition,
whereas, the spin-glass like state forms due to the existence of anti-site
disorder intrinsic to the material.Comment: 5 pages, 4 figures, supplementary material included in a separate
file; accepted for publication in PR
Strain and order-parameter coupling in Ni-Mn-Ga Heusler alloys from resonant ultrasound spectroscopy
Resonant ultrasound spectroscopy and magnetic susceptibility experiments have
been used to characterize strain coupling phenomena associated with structural
and magnetic properties of the shape-memory Heusler alloy series
NiMnGa (, 2.5, 5.0, and 7.5). All samples exhibit
a martensitic transformation at temperature and ferromagnetic ordering at
temperature , while the pure end member () also has a premartensitic
transition at , giving four different scenarios: ,
without premartensitic transition, , and .
Fundamental differences in elastic properties i.e., stiffening versus
softening, are explained in terms of coupling of shear strains with three
discrete order parameters relating to magnetic ordering, a soft mode and the
electronic instability responsible for the large strains typical of martensitic
transitions. Linear-quadratic or biquadratic coupling between these order
parameters, either directly or indirectly via the common strains, is then used
to explain the stabilities of the different structures. Acoustic losses are
attributed to critical slowing down at the premartensite transition, to the
mobility of interphases between coexisting phases at the martensitic transition
and to mobility of some aspect of the twin walls under applied stress down to
the lowest temperatures at which measurements were made.Comment: 9 pages, 5 figure
Direct measurements of the magnetocaloric effect in pulsed magnetic fields: The example of the Heusler alloy NiMnIn
We have studied the magnetocaloric effect (MCE) in the shape-memory Heusler
alloy NiMnIn by direct measurements in pulsed magnetic
fields up to 6 and 20 T. The results in 6 T are compared with data obtained
from heat-capacity experiments. We find a saturation of the inverse MCE,
related to the first-order martensitic transition, with a maximum adiabatic
temperature change of K at 250 K and a conventional
field-dependent MCE near the second-order ferromagnetic transition in the
austenitic phase. The pulsed magnetic field data allow for an analysis of the
temperature response of the sample to the magnetic field on a time scale of
to 100 ms which is on the order of typical operation frequencies (10
to 100 Hz) of magnetocaloric cooling devices. Our results disclose that in
shape-memory alloys the different contributions to the MCE and hysteresis
effects around the martensitic transition have to be carefully considered for
future cooling applications.Comment: 5 pages, 4 figure
1995, Spatial and temporal variability of late Neogene equatorial Pacific carbonate
High-resolution, continuous records of GRAPE wet bulk density (a carbonate proxy) from Ocean Drilling Program Leg 138 provide one the opportunity for a detailed study of eastern equatorial Pacific Ocean carbonate sedimentation during the last 6 m.y. The transect of sites drilled spans both latitude and longitude in the eastern equatorial Pacific from 90° to 110°W and from 5°S to 10°N. Two modes of variability are resolved through the use of Empirical Orthogonal Function (EOF) analysis. In the presence of large tectonic and climatic boundary condition changes over the last 6 m.y., the dominant mode of spatial variability in carbonate sedimentation is remarkably constant. The first mode accounts for over 50% of the variance in the data, and is consistent with forcing by equatorial divergence. This mode characterizes both carbonate concentration and carbonate mass accumulation rate time series. Variability in the first mode is highly coherent with insolation, indicating a strong linear relationship between equatorial Pacific car bonate sedimentation and Milankovitch variability. Frequency domain analysis indicates that the coupling to equatorial divergence in carbonate sedimentation is strongest in the precession band (19-23 k.y.) and weakest though present at lower frequencies. The second mode of variability has a consistent spatial pattern of east-west asymmetry over the past 4 m.y. only; prior to 4 Ma, a different mode of spatial variability may have been present, possibly suggesting influence by closure of the Isthmus of Panama or other tectonic changes. The second mode of variability may indicate influence by CaCO3 dissolution. The second mode of variability is not highly coherent with insolation. Comparison of the modes of carbonate variability to a 4 m.y. record of benthic δ 1 8 indicates that although overall correlation between carbonate and δ 1 8 is low, both modes of variability in carbonate sedimentation are coherent with δ 1 8 changes at some frequencies. The first mode of carbonate variability is coherent with Sites 846/849 δ 1 8 at the dominant insolation periods, and the second mode is coherent at 100 k.y. during the last 2 m.y. The coherence between carbonate sedimentation and δ 1 8 in both EOF modes suggests that multiple uncorrelated modes of variability operated within the climate system during the late Neogene
Upper critical field of CeCoIn5
We present a detailed analysis of the upper critical field for CeCoIn5 under
high pressure. We show that, consistently with other measurements, this system
shows a decoupling between maximum of the superconducting transition
temperature Tc and maximum pairing strength. This puts forward CeCoIn5 as an
important paradigm for this class of unconventional, strongly correlated
superconductors.Comment: 15 pages, 5 figures, author version, published in New J. Phy
A precursor state to unconventional superconductivity in CeIrIn
We present sensitive measurements of the Hall effect and magnetoresistance in
CeIrIn down to temperatures of 50 mK and magnetic fields up to 15 T. The
presence of a low temperature coherent Kondo state is established. Deviations
from Kohler's rule and a quadratic temperature dependence of the cotangent of
the Hall angle are reminiscent of properties observed in the high temperature
superconducting cuprates. The most striking observation pertains to the
presence of a \textit{precursor} state--characterized by a change in the Hall
mobility--that appears to precede the superconductivity in this material, in
similarity to the pseudogap in the cuprate high superconductors.Comment: 4 figure
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