678 research outputs found
Performance constraints and compensation for teleoperation with delay
A classical control perspective is used to characterize performance constraints and evaluate compensation techniques for teleoperation with delay. Use of control concepts such as open and closed loop performance, stability, and bandwidth yield insight to the delay problem. Teleoperator performance constraints are viewed as an open loop time delay lag and as a delay-induced closed loop bandwidth constraint. These constraints are illustrated with a simple analytical tracking example which is corroborated by a real time, 'man-in-the-loop' tracking experiment. The experiment also provides insight to those controller characteristics which are unique to a human operator. Predictive displays and feedforward commands are shown to provide open loop compensation for delay lag. Low pass filtering of telemetry or feedback signals is interpreted as closed loop compensation used to maintain a sufficiently low bandwidth for stability. A new closed loop compensation approach is proposed that uses a reactive (or force feedback) hand controller to restrict system bandwidth by impeding operator inputs
An ab-initio theoretical investigation of the soft-magnetic properties of permalloys
We study Ni80Fe20-based permalloys with the relativistic spin-polarized
Korringa-Kohn-Rostoker electronic structure method. Treating the compositional
disorder with the coherent potential approximation, we investigate how the
magnetocrystalline anisotropy, K, and magnetostriction, lambda, of Ni-rich
Ni-Fe alloys vary with the addition of small amounts of non-magnetic transition
metals, Cu and Mo. From our calculations we follow the trends in K and lambda
and find the compositions of Ni-Fe-Cu and Ni-Fe-Mo where both are near zero.
These high permeability compositions of Ni-Fe-Cu and Ni-Fe-Mo match well with
those discovered experimentally. We monitor the connection of the magnetic
anisotropy with the number of minority spin electrons, Nmin. By raising Nmin
via artificially increasing the band-filling of Ni80Fe20, we are able to
reproduce the key features that underpin the magnetic softening we find in the
ternary alloys. The effect of band-filling on the dependence of
magnetocrystalline anisotropy on atomic short-range order in Ni80Fe20 is also
studied. Our calculations, based on a static concentration wave theory,
indicate that the susceptibility of the high permeability of the Ni-Fe-Cu and
Ni-Fe-Mo alloys to their annealing conditions is also strongly dependent on the
alloys' compositions. An ideal soft magnet appears from these calculations.Comment: 20 pages, 6 figure
Mangetic properties of Ising thin-films with cubic lattices
We have used Monte Carlo simulations to observe the magnetic behaviour of
Ising thin-films with cubic lattice structures as a function of temperature and
thickness especially in the critical region. The fourth order Binder cumulant
is used to extract critical temperatures, and an extension of finite size
scaling theory for reduced geometry is derived to calculate the critical
exponents. Magnetisation and magnetic susceptibility per spin in each layer are
also investigated. In addition, mean-field calculations are also performed for
comparison. We find that the magnetic behaviour changes from two dimensional to
three dimensional character with increasing thickness of the film. The
crossover of the critical temperature from a two dimensional to a bulk value is
also observed with both the Monte Carlo simulations and the mean-field
analysis. Nevertheless, the simulations have shown that the critical exponents
only vary a little from their two dimensional values. In particular, the
results for films with up to eight layers provide a strong indication of two
dimensional universality.Comment: 18 pages, 9 figures, 2 table
Fluctuating local moments, itinerant electrons and the magnetocaloric effect: the compositional hypersensitivity of FeRh
We describe an ab-initio Disordered Local Moment Theory for materials with
quenched static compositional disorder traversing first order magnetic phase
transitions. It accounts quantitatively for metamagnetic changes and the
magnetocaloric effect. For perfect stoichiometric B2-ordered FeRh, we calculate
the transition temperature of the ferromagnetic-antiferromagnetic transition to
be 495K and a maximum isothermal entropy change in 2 Tesla of J~K~kg. A large (40\%) component of is
electronic. The transition results from a fine balance of competing electronic
effects which is disturbed by small compositional changes - e.g. swapping just
2\% Fe of `defects' onto the Rh sublattice makes drop by 290K. This
hypersensitivity explains the narrow compositional range of the transition and
impurity doping effects.Comment: 11 pages, 4 figure
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