Learning Complex Motor Skills for Legged Robot Fall Recovery

Falling is inevitable for legged robots in challenging real-world scenarios, where environments are unstructured and situations are unpredictable, such as uneven terrain in the wild. Hence, to recover from falls and achieve all-terrain traversability, it is essential for intelligent robots to possess the complex motor skills required to resume operation. To go beyond the limitation of handcrafted control, we investigated a deep reinforcement learning approach to learn generalized feedback-control policies for fall recovery that are robust to external disturbances. We proposed a design guideline for selecting key states for initialization, including a comparison to the random state initialization. The proposed learning-based pipeline is applicable to different robot models and their corner cases, including both small-/large-size bipeds and quadrupeds. Further, we show that the learned fall recovery policies are hardware-feasible and can be implemented on real robots

Interactions of energetic electrons with ULF waves triggered by interplanetary shock: Van Allen Probes observations in the magnetotail

Abstract We present in situ observations of a shock-induced substorm-like event on 13 April 2013 observed by the newly launched Van Allen twin probes. Substorm-like electron injections with energy of 30-500 keV were observed in the region from L∼5.2 to 5.5 immediately after the shock arrival (followed by energetic electron drift echoes). Meanwhile, the electron flux was clearly and strongly varying on the ULF wave time scale. It is found that both toroidal and poloidal mode ULF waves with a period of 150 s emerged following the magnetotail magnetic field reconfiguration after the interplanetary (IP) shock passage. The poloidal mode is more intense than the toroidal mode. The 90 phase shift between the poloidal mode Br and Ea suggests the standing poloidal waves in the Northern Hemisphere. Furthermore, the energetic electron flux modulations indicate that the azimuthal wave number is ∼14. Direct evidence of drift resonance between the injected electrons and the excited poloidal ULF wave has been obtained. The resonant energy is estimated to be between 150 keV and 230 keV. Two possible scenaria on ULF wave triggering are discussed: vortex-like flow structure-driven field line resonance and ULF wave growth through drift resonance. It is found that the IP shock may trigger intense ULF wave and energetic electron behavior at L∼3 to 6 on the nightside, while the time profile of the wave is different from dayside cases

Surface integrity in dry and cryogenic machining of AZ31B Mg alloy with varying cutting edge radius tools

Surface integrity of machined products has a critical impact on their functional performance. Magnesium alloys are lightweight materials for transportation industry and are also emerging as a potential material for temporary biomedical implants. However, their unsatisfactory corrosion resistance limits their application to a great extent. Surface integrity factors, such as grain size, crystallographic orientation and residual stresses, were reported to have significant influence on corrosion resistance of AZ31 Mg alloys. In this study, AZ31B Mg discs were orthogonally turned using cutting tools with two edge radii under both dry and cryogenic conditions. The influence of cutting edge radius and cooling method on surface integrity was investigated. Cryogenic machining using a large edge radius tool led to a thicker grain refinement layer, larger compressive residual stresses and stronger intensity of basal texture, which may remarkably enhance the corrosion performance of magnesium alloys.info:eu-repo/semantics/publishedVersio

Integrable impurities in Hubbard chain with the open boundary condition

The Kondo problem of two impurities in 1D strongly correlated electron system within the framework of the open boundary Hubbard chain is solved and the impurities, coupled to the ends of the electron system, are introduced by their scattering matrices with electrons so that the boundary matrices satisfy the reflecting integrability condition. The finite size correction of the ground state energy is obtained due to the impurities. Exact expressions for the low temperature specific heat contributed by the charge and spin parts of the magnetic impurities are derived. The Pauli susceptibility and the Kondo temperature are given explicitly. The Kondo temperature is inversely proportional to the density of electrons.Comment: 6 pages, Revtex, To appear in Europhysics Letter

Pulmonary fissure integrity and collateral ventilation in COPD patients

Purpose: To investigate whether the integrity (completeness) of pulmonary fissures affects pulmonary function in patients with chronic obstructive pulmonary disease (COPD). Materials and Methods: A dataset consisting of 573 CT exams acquired on different subjects was collected from a COPD study. According to the global initiative for chronic obstructive lung disease (GOLD) criteria, these subjects (examinations) were classified into five different subgroups, namely non-COPD (222 subjects), GOLD-I (83 subjects), GOLD-II (141 subjects), GOLD-III (63 subjects), and GOLD-IV (64 subjects), in terms of disease severity. An available computer tool was used to aid in an objective and efficient quantification of fissure integrity. The correlations between fissure integrity, and pulmonary functions (e.g., FEV1, and FEV1/FVC) and COPD severity were assessed using Pearson and Spearman's correlation coefficients, respectively. Results: For the five sub-groups ranging from non-COPD to GOLD-IV, the average integrities of the right oblique fissure (ROF) were 81.8%, 82.4%, 81.8%, 82.8%, and 80.2%, respectively; the average integrities of the right horizontal fissure (RHF) were 62.6%, 61.8%, 62.1%, 62.2%, and 62.3%, respectively; the average integrities of the left oblique fissure (LOF) were 82.0%, 83.2%, 81.7%, 82.0%, and 78.4%, respectively; and the average integrities of all fissures in the entire lung were 78.0%, 78.6%, 78.1%, 78.5%, and 76.4%, respectively. Their Pearson correlation coefficients with FEV1 and FE1/FVC range from 0.027 to 0.248 with p values larger than 0.05. Their Spearman correlation coefficients with COPD severity except GOLD-IV range from -0.013 to -0.073 with p values larger than 0.08. Conclusion: There is no significant difference in fissure integrity for patients with different levels of disease severity, suggesting that the development of COPD does not change the completeness of pulmonary fissures and incomplete fissures alone may not contribute to the collateral ventilation. © 2014 Pu et al

Hydrographic observations from the US/PRC Cooperative Program in the Western Equatorial Pacific Ocean, cruises 5-8

In support of the Tropical Ocean and Global Atmosphere (TOGA) program, investigators from the Woods Hole Oceanographic Institution (WHOI), NOAA Pacific Marine Environmental Laboratory (PMEL), and the State Oceanic Administration (SOA) from both Qingdao (First Institute) and Guangzhou (South China Sea Branch) conducted hydrographic observations aboard the Chinese R/V Xiang Yang Hong 14 in the western equatorial Pacific Ocean. The objective of this component of the TOGA program was to document the water mass property distributions of the western equatorial Pacific and describe the oceanic velocity field. The four cruises summarized here were conducted during the period November 1988 to July 1990 and are the final half of an eight cruise repeated survey of the region begun in 1985. Conductivity-Temperature-Depth-Oxygen (CTD/O2) stations were collected to a minimum cast depth of 2500m or the bottom when shallower. The cruises reoccupied the same stations to provide temporal information. Summarized listings of CTD/02 data together with selected physical properties of sea water for these cruises are provided here, as well as a description of the hardware used and an explanation of the data reduction techniques employed.Funding was provided by the National Oceanic and Atmospheric Administration under Grant No. NA85AA-D-AC117