484 research outputs found
Trajectory generation for multi-contact momentum-control
Simplified models of the dynamics such as the linear inverted pendulum model
(LIPM) have proven to perform well for biped walking on flat ground. However,
for more complex tasks the assumptions of these models can become limiting. For
example, the LIPM does not allow for the control of contact forces
independently, is limited to co-planar contacts and assumes that the angular
momentum is zero. In this paper, we propose to use the full momentum equations
of a humanoid robot in a trajectory optimization framework to plan its center
of mass, linear and angular momentum trajectories. The model also allows for
planning desired contact forces for each end-effector in arbitrary contact
locations. We extend our previous results on LQR design for momentum control by
computing the (linearized) optimal momentum feedback law in a receding horizon
fashion. The resulting desired momentum and the associated feedback law are
then used in a hierarchical whole body control approach. Simulation experiments
show that the approach is computationally fast and is able to generate plans
for locomotion on complex terrains while demonstrating good tracking
performance for the full humanoid control
Reach and speed of judgment propagation in the laboratory
In recent years, a large body of research has demonstrated that judgments and
behaviors can propagate from person to person. Phenomena as diverse as
political mobilization, health practices, altruism, and emotional states
exhibit similar dynamics of social contagion. The precise mechanisms of
judgment propagation are not well understood, however, because it is difficult
to control for confounding factors such as homophily or dynamic network
structures. We introduce a novel experimental design that renders possible the
stringent study of judgment propagation. In this design, experimental chains of
individuals can revise their initial judgment in a visual perception task after
observing a predecessor's judgment. The positioning of a very good performer at
the top of a chain created a performance gap, which triggered waves of judgment
propagation down the chain. We evaluated the dynamics of judgment propagation
experimentally. Despite strong social influence within pairs of individuals,
the reach of judgment propagation across a chain rarely exceeded a social
distance of three to four degrees of separation. Furthermore, computer
simulations showed that the speed of judgment propagation decayed exponentially
with the social distance from the source. We show that information distortion
and the overweighting of other people's errors are two individual-level
mechanisms hindering judgment propagation at the scale of the chain. Our
results contribute to the understanding of social contagion processes, and our
experimental method offers numerous new opportunities to study judgment
propagation in the laboratory
Hyperfine Splitting and the Zeeman Effect in Holographic Heavy-Light Mesons
We inspect the mass spectrum of heavy-light mesons in deformed N=2 super
Yang-Mills theory using the AdS/CFT correspondence. We demonstrate how some of
the degeneracies of the supersymmetric meson spectrum can be removed upon
breaking the supersymmetry, thus leading to the emergence of hyperfine
structure. The explicit SUSY breaking scenarios we consider involve on one hand
tilting one of the two fundamental D7 branes inside the internal R^6 space, and
on the other hand applying an external magnetic field on the (untilted) branes.
The latter scenario leads to the well-known Zeeman effect, which we inspect for
both weak and strong magnetic fields.Comment: 5 pages, 1 figur
Humanoid Momentum Estimation Using Sensed Contact Wrenches
This work presents approaches for the estimation of quantities important for
the control of the momentum of a humanoid robot. In contrast to previous
approaches which use simplified models such as the Linear Inverted Pendulum
Model, we present estimators based on the momentum dynamics of the robot. By
using this simple yet dynamically-consistent model, we avoid the issues of
using simplified models for estimation. We develop an estimator for the center
of mass and full momentum which can be reformulated to estimate center of mass
offsets as well as external wrenches applied to the robot. The observability of
these estimators is investigated and their performance is evaluated in
comparison to previous approaches.Comment: Submitted to the 15th IEEE RAS Humanoids Conference, to be held in
Seoul, Korea on November 3 - 5, 201
Balancing experiments on a torque-controlled humanoid with hierarchical inverse dynamics
Recently several hierarchical inverse dynamics controllers based on cascades
of quadratic programs have been proposed for application on torque controlled
robots. They have important theoretical benefits but have never been
implemented on a torque controlled robot where model inaccuracies and real-time
computation requirements can be problematic. In this contribution we present an
experimental evaluation of these algorithms in the context of balance control
for a humanoid robot. The presented experiments demonstrate the applicability
of the approach under real robot conditions (i.e. model uncertainty, estimation
errors, etc). We propose a simplification of the optimization problem that
allows us to decrease computation time enough to implement it in a fast torque
control loop. We implement a momentum-based balance controller which shows
robust performance in face of unknown disturbances, even when the robot is
standing on only one foot. In a second experiment, a tracking task is evaluated
to demonstrate the performance of the controller with more complicated
hierarchies. Our results show that hierarchical inverse dynamics controllers
can be used for feedback control of humanoid robots and that momentum-based
balance control can be efficiently implemented on a real robot.Comment: appears in IEEE/RSJ International Conference on Intelligent Robots
and Systems (IROS), 201
On Time Optimization of Centroidal Momentum Dynamics
Recently, the centroidal momentum dynamics has received substantial attention
to plan dynamically consistent motions for robots with arms and legs in
multi-contact scenarios. However, it is also non convex which renders any
optimization approach difficult and timing is usually kept fixed in most
trajectory optimization techniques to not introduce additional non convexities
to the problem. But this can limit the versatility of the algorithms. In our
previous work, we proposed a convex relaxation of the problem that allowed to
efficiently compute momentum trajectories and contact forces. However, our
approach could not minimize a desired angular momentum objective which
seriously limited its applicability. Noticing that the non-convexity introduced
by the time variables is of similar nature as the centroidal dynamics one, we
propose two convex relaxations to the problem based on trust regions and soft
constraints. The resulting approaches can compute time-optimized dynamically
consistent trajectories sufficiently fast to make the approach realtime
capable. The performance of the algorithm is demonstrated in several
multi-contact scenarios for a humanoid robot. In particular, we show that the
proposed convex relaxation of the original problem finds solutions that are
consistent with the original non-convex problem and illustrate how timing
optimization allows to find motion plans that would be difficult to plan with
fixed timing.Comment: 7 pages, 4 figures, ICRA 201
Momentum Control with Hierarchical Inverse Dynamics on a Torque-Controlled Humanoid
Hierarchical inverse dynamics based on cascades of quadratic programs have
been proposed for the control of legged robots. They have important benefits
but to the best of our knowledge have never been implemented on a torque
controlled humanoid where model inaccuracies, sensor noise and real-time
computation requirements can be problematic. Using a reformulation of existing
algorithms, we propose a simplification of the problem that allows to achieve
real-time control. Momentum-based control is integrated in the task hierarchy
and a LQR design approach is used to compute the desired associated closed-loop
behavior and improve performance. Extensive experiments on various balancing
and tracking tasks show very robust performance in the face of unknown
disturbances, even when the humanoid is standing on one foot. Our results
demonstrate that hierarchical inverse dynamics together with momentum control
can be efficiently used for feedback control under real robot conditions.Comment: 21 pages, 11 figures, 4 tables in Autonomous Robots (2015
Mediation centrality in adversarial policy networks
Conflict resolution often involves mediators who understand the issues central to both sides of an argument. Mediators in complex networks represent nodes that are connected to other key nodes in opposing subgraphs. Here we introduce a new metric, mediation centrality, for iden- tifying good mediators in adversarial policy networks, such as the connections between indi- viduals and their reasons for and against the support of controversial topics (e.g., state-financed abortion). Using a process-based account of reason mediation we construct bipartite adversar- ial policy networks and show how mediation defined over subgraph projections constrained to reasons representing opposing sides can be used to produce a measure of mediation centrality that is superior to centrality computed on the full network. We then empirically illustrate and test mediation centrality in a “policy fluency task,” where participants generated reasons for or against eight controversial policy issues (state-subsidized abortion, bank bailouts, forced CO2 reduction, cannabis legalization, shortened naturalization, surrogate motherhood legalization, public smoking ban, and euthanasia legalization). We discuss how mediation centrality can be extended to adversarial policy networks with more than two positions and to other centrality measures
Crosslinking-MS analysis reveals RNA polymerase I domain architecture and basis of rRNA cleavage
RNA polymerase (Pol) I contains a 10-subunit catalytic core that is related to the core of Pol II and includes subunit A12.2. In addition, Pol I contains the heterodimeric subcomplexes A14/43 and A49/34.5, which are related to the Pol II subcomplex Rpb4/7 and the Pol II initiation factor TFIIF, respectively. Here we used lysine-lysine crosslinking, mass spectrometry (MS) and modeling based on five crystal structures, to extend the previous homology model of the Pol I core, to confirm the location of A14/43 and to position A12.2 and A49/34.5 on the core. In the resulting model of Pol I, the C-terminal ribbon (C-ribbon) domain of A12.2 reaches the active site via the polymerase pore, like the C-ribbon of the Pol II cleavage factor TFIIS, explaining why the intrinsic RNA cleavage activity of Pol I is strong, in contrast to the weak cleavage activity of Pol II. The A49/34.5 dimerization module resides on the polymerase lobe, like TFIIF, whereas the A49 tWH domain resides above the cleft, resembling parts of TFIIE. This indicates that Pol I and also Pol III are distantly related to a Pol II-TFIIS-TFIIF-TFIIE comple
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