78,732 research outputs found
Adaptive and Supertwisting Adaptive Spacecraft Orbit Control Around Asteroids
This paper addresses the development of control systems for the orbit control of spacecraft around irregularly shaped rotating asteroids with uncertain parameters. The objective is to steer the spacecraft along prescribed orbits. First, a nonlinear adaptive law for orbit control was designed. This was followed by the design of a supertwisting adaptive (STWA) control system. In the closed-loop system, which includes the adaptive law or the STWA law, all the signals remain bounded, and the trajectory tracking error asymptotically converges to zero for any initial condition. Finally, under the assumption of boundedness of the derivative of the uncertain functions of the model in a region of the state space, a supertwisting control (STW) law for finite-time convergence of the trajectory was obtained. Based on the Lyapunov theory, stability properties of the closed-loop systems were analyzed. Simulation results for 433 Eros and Ida asteroids were presented for illustration. The results showed that control of spacecraft along closed orbits or to a fixed point is accomplished using each of these controllers, despite uncertainties in the parameters of the asteroid models
From quantum pulse gate to quantum pulse shaper -- enigneered frequency conversion in nonlinear optical waveguides
Full control over the spatio-temporal structure of quantum states of light is
an important goal in quantum optics, to generate for instance single-mode
quantum pulses or to encode information on multiple modes, enhancing channel
capacities. Quantum light pulses feature an inherent, rich spectral
broadband-mode structure. In recent years, exploring the use of integrated
optics as well as source-engineering has led to a deep understanding of the
pulse-mode structure of guided quantum states of light. In addition, several
groups have started to investigate the manipulation of quantum states by means
of single-photon frequency conversion. In this paper we explore new routes
towards complete control of the inherent pulse-modes of ultrafast pulsed
quantum states by employing specifically designed nonlinear waveguides with
adapted dispersion properties. Starting from our recently proposed quantum
pulse gate (QPG) we further generalize the concept of spatio-spectral
engineering for arbitrary \chitwo-based quantum processes. We analyse the
sum-frequency generation based QPG and introduce the difference-frequency
generation based quantum pulse shaper (QPS). Together, these versatile and
robust integrated optics devices allow for arbitrary manipulations of the
pulse-mode structure of ultrafast pulsed quantum states. The QPG can be
utilized to select an arbitrary pulse mode from a multimode input state,
whereas the QPS enables the generation of specific pulse modes from an input
wavepacket with Gaussian-shaped spectrum.Comment: 21 pages, 9 figure
Ultra-Sparse Non-Binary LDPC Codes for Probabilistic Amplitude Shaping
This work shows how non-binary low-density parity-check codes over GF()
can be combined with probabilistic amplitude shaping (PAS) (B\"ocherer, et al.,
2015), which combines forward-error correction with non-uniform signaling for
power-efficient communication. Ultra-sparse low-density parity-check codes over
GF(64) and GF(256) gain 0.6 dB in power efficiency over state-of-the-art binary
LDPC codes at a spectral efficiency of 1.5 bits per channel use and a
blocklength of 576 bits. The simulation results are compared to finite length
coding bounds and complemented by density evolution analysis.Comment: Accepted for Globecom 201
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Sliding mode and shaped input vibration control of flexible systems
Copyright [2008] IEEE. This material is posted here with permission of the IEEE. Such permission of the IEEE does not in any way imply IEEE endorsement of any of Brunel University's products or services. Internal or personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution must be obtained from the IEEE by writing to [email protected]. By choosing to view this document, you agree to all provisions of the copyright laws protecting it.In this paper, the vibration reduction problem is investigated for a flexible spacecraft during attitude maneuvering. A new control strategy is proposed, which integrates both the command input shaping and the sliding mode output feedback control (SMOFC) techniques. Specifically, the input shaper is designed for the reference model and implemented outside of the feedback loop in order to achieve the exact elimination of the residual vibration by modifying the existing command. The feedback controller, on the other hand, is designed based on the SMOFC such that the closed-loop system behaves like the reference model with input shaper, where the residual vibrations are eliminated in the presence of parametric uncertainties and external disturbances. An attractive feature of this SMOFC algorithm is that the parametric uncertainties or external disturbances of the system do not need to satisfy the so-called matching conditions or invariance conditions provided that certain bounds are known. In addition, a smoothed hyperbolic tangent function is introduced to eliminate the chattering phenomenon. Compared with the conventional methods, the proposed scheme guarantees not only the stability of the closed-loop system, but also the good performance as well as the robustness. Simulation results for the spacecraft model show that the precise attitudes control and vibration suppression are successfully achieved
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Numerical Simulation of the Temperature Distribution and Solid-Phase Evolution in the LENS™ Process
A three-dimensional finite element model was developed and applied to analyze the
temperature and phase evolution in deposited stainless steel 410 (SS410) during the Laser
Engineered Net Shaping (LENSTM) rapid fabrication process. The effect of solid phase
transformations is taken into account by using temperature and phase dependent material
properties and the continuous cooling transformation (CCT) diagram. The laser beam is modeled
as a Gaussian distribution of heat flux from a moving heat source with conical shape. The laser
power is optimized in order to achieve a pre-defined molten pool size for each layer. It is found
that approximately 5% decrease of the laser power for each pass is required to obtain a steady
molten pool size. The temperature distribution and cooling rate surrounding the molten pool are
predicted and compared with experiments. Based upon the predicted thermal cycles and cooling
rate, the phase transformations and their effects on the hardness are discussed.Mechanical Engineerin
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