1,126 research outputs found
Hierarchies of Planning and Reinforcement Learning for Robot Navigation
Solving robotic navigation tasks via reinforcement learning (RL) is
challenging due to their sparse reward and long decision horizon nature.
However, in many navigation tasks, high-level (HL) task representations, like a
rough floor plan, are available. Previous work has demonstrated efficient
learning by hierarchal approaches consisting of path planning in the HL
representation and using sub-goals derived from the plan to guide the RL policy
in the source task. However, these approaches usually neglect the complex
dynamics and sub-optimal sub-goal-reaching capabilities of the robot during
planning. This work overcomes these limitations by proposing a novel
hierarchical framework that utilizes a trainable planning policy for the HL
representation. Thereby robot capabilities and environment conditions can be
learned utilizing collected rollout data. We specifically introduce a planning
policy based on value iteration with a learned transition model (VI-RL). In
simulated robotic navigation tasks, VI-RL results in consistent strong
improvement over vanilla RL, is on par with vanilla hierarchal RL on single
layouts but more broadly applicable to multiple layouts, and is on par with
trainable HL path planning baselines except for a parking task with difficult
non-holonomic dynamics where it shows marked improvements.Comment: 7 pages, 5 figures, 2021 IEEE International Conference on Robotics
and Automation (ICRA), v2: DOI number adde
The very fast evolution of Sakurai's object
V4334 Sgr (a.k.a. Sakurai's object) is the central star of an old planetary
nebula that underwent a very late thermal pulse a few years before its
discovery in 1996. We have been monitoring the evolution of the optical
emission line spectrum since 2001. The goal is to improve the evolutionary
models by constraining them with the temporal evolution of the central star
temperature. In addition the high resolution spectral observations obtained by
X-shooter and ALMA show the temporal evolution of the different morphological
components.Comment: 2 pages, 2 figures to appear in the Proceedings of the IAU Symp. 323:
"Planetary nebulae: Multi-wavelength probes of stellar and galactic
evolution". Eds. X.-W. Liu, L. Stanghellini and A. Karaka
Why should i comply? Sellers' accounts for (non-)compliance with legal age limits for alcohol sales
Background\ud
Availability is an important predictor of early and excessive alcohol consumption by adolescents. Many countries have implemented age limits to prevent underage purchases of alcohol. However, shop-floor compliance with these age limits appears to be problematic. This study addresses the issue of non-compliance with age limits. Which measures do vendors take to avoid underage alcohol sales, and what do they report as important reasons to comply or not with age limits for alcohol sales? \ud
\ud
Methods\ud
Open-ended telephone interviews were conducted with store managers selling alcohol (N = 106). Prior to the interviews, all outlets were visited by an underage mystery shopper in order to measure compliance with the legal age limits on alcohol sales. The interview results are compared against actual compliance rates. \ud
\ud
Results\ud
Several measures have been taken to prevent underage sales, but the compliance level is low. Furthermore, open coding resulted in 19 themes, representing both valid and invalid arguments, that vendors mentioned as relevant to their decisions of whether to comply with the law. Compliance with age limits is dependent on the knowledge of the rules and the ability and motivation to follow the rules. The ability aspect in particular seems to be problematic, but in many cases, the motivation to actively comply with the age limits is lacking. \ud
\ud
Conclusions\ud
To enhance compliance, it is important to raise the awareness of the importance of age limits and to connect possible violations of the regulations to negative consequences\u
The onset of photoionization in Sakurai's Object (V4334 Sgr)
We investigate the reheating of the very late thermal pulse (VLTP) object
V4334 Sgr (Sakurai's Object) using radio observations from the Very Large
Array, and optical spectra obtained with the Very Large Telescope. We find a
sudden rise of the radio flux at 5 and 8 GHz - from <= 90 micro-Jy and 80 +/-
30 micro-Jy in February 2005 to 320 micro-Jy and 280 micro-Jy in June 2006.
Optical line emission is also evolving, but the emission lines are fading. The
optical line emission and early radio flux are attributed to a fast shock (and
not photoionization as was reported earlier) which occurred around 1998. The
fading is due to post-shock cooling and recombination. The recent rapid
increase in radio flux is evidence for the onset of photoionization of carbon
starting around 2005. The current results indicate an increase in the stellar
temperature to 12 kK in 2006. The mass ejected in the VLTP eruption is M_ej >=
1e-4 Msol, but could be as high as 1e-2 Msol, depending mainly on the distance
and the clumping factor of the outflow. We derive a distance between 1.8 and 5
kpc. A high mass loss could expose the helium layer and yield abundances
compatible with those of [WC] and PG1159 stars.Comment: 4 pages, 2 figures; accepted for publication in A&A letter
- âŠ