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1.
This paper addresses the combined problem of trajectory planning and tracking control for underactuated autonomous underwater vehicles (AUVs) on the horizontal plane. Given a smooth, inertial, 2D reference trajectory, the planning algorithm uses vehicle dynamics to compute the reference orientation and body-fixed velocities. Using these, the error dynamics are obtained. These are stabilized using backstepping techniques, forcing the tracking error to an arbitrarily small neighborhood of zero. Simulation results for a constant velocity trajectory, i.e. a circle, and a time-varying velocity one, i.e. a sinusoidal path, are presented. The parametric robustness is considered and it is shown that tracking remains satisfactory. 相似文献
2.
M. R. Kundu 《Journal of Earth System Science》1984,93(3):201-225
We review high spatial resolution microwave observations of solar active regions, coronal loops and flares. Observations of
preflare active regions are presented; in particular we discuss the interpretations of reversal of polarization at the flare
site and the role of newly emerging flux in triggering the onset of flares. We discuss the spatial locations of microwave
burst emitting regions; loops or arcades of loops appear to be the sites of flare energy release in microwave bursts. We provide
direct observational evidence of magnetic reconnection as the primary cause of acceleration of electrons in microwave bursts. 相似文献
3.
本文给出了１９８９ 年８ 月１７ 日耀斑后环的观测视向速度场,在环内物质在太阳重力、磁场梯度和大气压力梯度联合作用下沿环腿螺旋上升和环内物质密度由环腿向环顶和环足线性递增的假设下,理论上计算了该环系的视向速度场,理论计算和观测结果基本相符,似乎为耀斑物质由色球蒸发作上升运动的观点提供了间接的例证 相似文献
4.
5.
6.
多径信号是GPS定位的主要误差源之一。码和载波跟踪环是GPS接收机的重要组成部分，它们直接决定了接收机的性能，因此当前多径消除技术研究的核心即是从跟踪环内部着手研究。本文仔细分析了GPS接收机信号跟踪环以及多径信号对跟踪精度的影响，对伪距码与载波相位多径误差进行了比较，并且通过仿真结果和图表阐明了码和载波相位多径误差之间的协同关系。 相似文献
7.
8.
With modern imaging and spectral instruments observing in the visible, EUV, X-ray, and radio wavelengths, the detection of
oscillations in the solar outer atmosphere has become a routine event. These oscillations are considered to be the signatures
of a wave phenomenon and are generally interpreted in terms of magnetohydrodynamic (MHD) waves. With multiwavelength observations
from ground- and space-based instruments, it has been possible to detect waves in a number of different wavelengths simultaneously
and, consequently, to study their propagation properties. Observed MHD waves propagating from the lower solar atmosphere into
the higher regions of the magnetized corona have the potential to provide excellent insight into the physical processes at
work at the coupling point between these different regions of the Sun. High-resolution wave observations combined with forward
MHD modeling can give an unprecedented insight into the connectivity of the magnetized solar atmosphere, which further provides
us with a realistic chance to reconstruct the structure of the magnetic field in the solar atmosphere. This type of solar
exploration has been termed atmospheric magnetoseismology. In this review we will summarize some new trends in the observational
study of waves and oscillations, discussing their origin and their propagation through the atmosphere. In particular, we will
focus on waves and oscillations in open magnetic structures (e.g., solar plumes) and closed magnetic structures (e.g., loops and prominences), where there have been a number of observational highlights in the past few years. Furthermore, we
will address observations of waves in filament fibrils allied with a better characterization of their propagating and damping
properties, the detection of prominence oscillations in UV lines, and the renewed interest in large-amplitude, quickly attenuated,
prominence oscillations, caused by flare or explosive phenomena. 相似文献
9.
A mechanism of damped oscillations of a coronal loop is investigated. The loop is treated as a thin toroidal flux rope with
two stationary photospheric footpoints, carrying both toroidal and poloidal currents. The forces and the flux-rope dynamics
are described within the framework of ideal magnetohydrodynamics (MHD). The main features of the theory are the following:
i) Oscillatory motions are determined by the Lorentz force that acts on curved current-carrying plasma structures and ii) damping is caused by drag that provides the momentum coupling between the flux rope and the ambient coronal plasma. The
oscillation is restricted to the vertical plane of the flux rope. The initial equilibrium flux rope is set into oscillation
by a pulse of upflow of the ambient plasma. The theory is applied to two events of oscillating loops observed by the Transition Region and Coronal Explorer (TRACE). It is shown that the Lorentz force and drag with a reasonable value of the coupling coefficient (c
d
) and without anomalous dissipation are able to accurately account for the observed damped oscillations. The analysis shows
that the variations in the observed intensity can be explained by the minor radial expansion and contraction. For the two
events, the values of the drag coefficient consistent with the observed damping times are in the range c
d
≈2 – 5, with specific values being dependent on parameters such as the loop density, ambient magnetic field, and the loop
geometry. This range is consistent with a previous MHD simulation study and with values used to reproduce the observed trajectories
of coronal mass ejections (CMEs). 相似文献
10.
Many underwater intervention tasks are today performed using manned submersibles or remotely operated vehicles in teleoperation mode. Autonomous underwater vehicles are mostly employed in survey applications. In fact, the low bandwidth and significant time delay inherent in acoustic subsea communications represent a considerable obstacle to remotely operate a manipulation system, making it impossible for remote controllers to react to problems in a timely manner.Nevertheless, vehicles with no physical link and with no human occupants permit intervention in dangerous areas, such as in deep ocean, under ice, in missions to retrieve hazardous objects, or in classified areas. The key element in underwater intervention performed with autonomous vehicles is autonomous manipulation. This is a challenging technology milestone, which refers to the capability of a robot system that performs intervention tasks requiring physical contacts with unstructured environments without continuous human supervision.Today, only few AUVs are equipped with manipulators. SAUVIM (Semi Autonomous Underwater Vehicle for Intervention Mission, University of Hawaii) is one of the first underwater vehicle capable of autonomous manipulation.This paper presents the solutions chosen within the development of the system in order to address the problems intrinsic to autonomous underwater manipulation. In the proposed approach, the most noticeable aspect is the increase in the level of information transferred between the system and the human supervisor.We describe one of the first trials of autonomous intervention performed by SAUVIM in the oceanic environment. To the best knowledge of the authors, no sea trials in underwater autonomous manipulation have been presented in the literature. The presented operation is an underwater recovery mission, which consists in a sequence of autonomous tasks finalized to search for the target and to securely hook a cable to it in order to bring the target to the surface. 相似文献