806 research outputs found
Stably Extending Two-Dimensional Bipedal Walking to Three Dimensions
In this paper we develop a feedback control law that results in stable walking gaits on flat ground for a three-dimensional bipedal robotic walker given stable walking gaits for a two-dimensional bipedal robotic walker. This is achieved by combining disparate techniques that have been employed in the bipedal robotic community: controlled symmetries, geometric reduction and hybrid zero dynamics. Controlled symmetries are utilized to obtain stable walking gaits for a two-dimensional bipedal robot walking on flat ground. These are related to walking gaits for a three-dimensional (hipless) bipedal robot through the use of geometric reduction. Finally, these walking gaits in three dimensions are made stable through the use of hybrid zero dynamics
A geometric approach to three-dimensional hipped bipedal robotic walking
This paper presents a control law that results in stable walking for a three-dimensional bipedal robot with a hip. To obtain this control law, we utilize techniques from geometric reduction, and specifically a variant of Routhian reduction termed functional Routhian reduction, to effectively decouple the dynamics of the three-dimensional biped into its sagittal and lateral components. Motivated by the decoupling afforded by functional Routhian reduction, the control law we present is obtained by combining three separate control laws: the first shapes the potential energy of the sagittal dynamics of the biped to obtain stable walking gaits when it is constrained to the sagittal plane, the second shapes the total energy of the walker so that functional Routhian reduction can be applied to decoupling the dynamics of the walker for certain initial conditions, and the third utilizes an output zeroing controller to stabilize to the surface defining these initial conditions. We numerically verify that this method results in stable walking, and we discuss certain attributes of this walking gait
The Dual-System Problem in Complex Conflicts
Conflict and fragile environments are increasingly complex and unpredictable, but the U.S. policy system itself is much more complex and unpredictable than most leaders appreciate. In this monograph, the authors argue that until we get a grasp on this “dual-system problem,” the United States will fall further and further behind in its strategic ambitions.https://press.armywarcollege.edu/monographs/1393/thumbnail.jp
The FIRST-2MASS Red Quasar Survey
Combining radio observations with optical and infrared color selection --
demonstrated in our pilot study to be an efficient selection algorithm for
finding red quasars -- we have obtained optical and infrared spectroscopy for
120 objects in a complete sample of 156 candidates from a sky area of 2716
square degrees. Consistent with our initial results, we find our selection
criteria -- J-K>1.7, R-K>4.0 -- yield a ~50% success rate for discovering
quasars substantially redder than those found in optical surveys. Comparison
with UVX- and optical color-selected samples shows that >~ 10% of the quasars
are missed in a magnitude-limited survey. Simultaneous two-frequency radio
observations for part of the sample indicate that a synchrotron continuum
component is ruled out as a significant contributor to reddening the quasars'
spectra. We go on to estimate extinctions for our objects assuming their red
colors are caused by dust. Continuum fits and Balmer decrements suggest E(B-V)
values ranging from near zero to 2.5 magnitudes. Correcting the K-band
magnitudes for these extinctions, we find that for K <= 14.0, red quasars make
up between 25% and 60% of the underlying quasar population; owing to the
incompleteness of the 2MASS survey at fainter K-band magnitudes, we can only
set a lower limit to the radio-detected red quasar population of >20-30%.Comment: 80 pages (single-column, preprint format) 20 figures, Accepted for
publicated in Ap
An FeLoBAL Binary Quasar
In an ongoing infrared imaging survey of quasars at Keck Observatory, we have
discovered that the z=1.285 quasar SDSS J233646.2-010732.6 comprises two point
sources with a separation of 1.67". Resolved spectra show that one component is
a standard quasar with a blue continuum and broad emission lines; the other is
a broad absorption line (BAL) quasar, specifically, a BAL QSO with prominent
absorption from MgII and metastable FeII, making it a member of the ``FeLoBAL''
class. The number of known FeLoBALs has recently grown dramatically from a
single example to more than a dozen, including a gravitationally lensed example
and the binary member presented here, suggesting that this formerly rare object
may be fairly common. Additionally, the presence of this BAL quasar in a
relatively small separation binary adds to the growing evidence that the BAL
phenomenon is not due to viewing a normal quasar at a specific orientation, but
rather that it is an evolutionary phase in the life of many, if not all,
quasars, and is particularly associated with conditions found in interacting
systems.Comment: AASTEX 13 pp., 4 figs; accepted by ApJ Letter
The Gravitational Lens Candidate FBQ 1633+3134
We present our ground-based optical imaging, spectral analysis, and high
resolution radio mapping of the gravitational lens candidate FBQ 1633+3134.
This z=1.52, B=17.7 quasar appears double on CCD images with an image
separation of 0.66 arcseconds and a flux ratio of ~3:1 across BVRI filters. A
single 0.27 mJy radio source is detected at 8.46 GHz, coincident to within an
arcsecond of both optical components, but no companion at radio wavelengths is
detected down to a flux level of 0.1 mJy (3 sigma). Spectral observations
reveal a rich metal-line absorption system consisting of a strong Mg II doublet
and associated Fe I and Fe II absorption features, all at an intervening
redshift of z=0.684, suggestive of a lensing galaxy. Point spread function
subtraction however shows no obvious signs of a third object between the two
quasar images, and places a detection limit of I > 23.0 if such an object
exists. Although the possibility that FBQ 1633+3134 is a binary quasar cannot
be ruled out, the evidence is consistent with it being a single quasar lensed
by a faint, metal-rich galaxy.Comment: 24 pages, 5 figures. Accepted by AJ. A calibration error affecting B
and V band apparent magnitudes has been corrected. The conclusions of the
paper are not change
Optimal Energy Shaping Control for a Backdrivable Hip Exoskeleton
Task-dependent controllers widely used in exoskeletons track predefined
trajectories, which overly constrain the volitional motion of individuals with
remnant voluntary mobility. Energy shaping, on the other hand, provides
task-invariant assistance by altering the human body's dynamic characteristics
in the closed loop. While human-exoskeleton systems are often modeled using
Euler-Lagrange equations, in our previous work we modeled the system as a
port-controlled-Hamiltonian system, and a task-invariant controller was
designed for a knee-ankle exoskeleton using interconnection-damping assignment
passivity-based control. In this paper, we extend this framework to design a
controller for a backdrivable hip exoskeleton to assist multiple tasks. A set
of basis functions that contains information of kinematics is selected and
corresponding coefficients are optimized, which allows the controller to
provide torque that fits normative human torque for different activities of
daily life. Human-subject experiments with two able-bodied subjects
demonstrated the controller's capability to reduce muscle effort across
different tasks
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