356 research outputs found
Analysis of optimal control problem formulations in skeletal movement predictions
Postprint (published version
Enhancing motion trajectory segmentation of rigid bodies using a novel screw-based trajectory-shape representation
Trajectory segmentation refers to dividing a trajectory into meaningful
consecutive sub-trajectories. This paper focuses on trajectory segmentation for
3D rigid-body motions. Most segmentation approaches in the literature represent
the body's trajectory as a point trajectory, considering only its translation
and neglecting its rotation. We propose a novel trajectory representation for
rigid-body motions that incorporates both translation and rotation, and
additionally exhibits several invariant properties. This representation
consists of a geometric progress rate and a third-order trajectory-shape
descriptor. Concepts from screw theory were used to make this representation
time-invariant and also invariant to the choice of body reference point. This
new representation is validated for a self-supervised segmentation approach,
both in simulation and using real recordings of human-demonstrated pouring
motions. The results show a more robust detection of consecutive submotions
with distinct features and a more consistent segmentation compared to
conventional representations. We believe that other existing segmentation
methods may benefit from using this trajectory representation to improve their
invariance.Comment: This work has been submitted to the IEEE International Conference on
Robotics and Automation (ICRA) for possible publication. Copyright may be
transferred without notice, after which this version may no longer be
accessibl
FATROP : A Fast Constrained Optimal Control Problem Solver for Robot Trajectory Optimization and Control
Trajectory optimization is a powerful tool for robot motion planning and
control. State-of-the-art general-purpose nonlinear programming solvers are
versatile, handle constraints in an effective way and provide a high numerical
robustness, but they are slow because they do not fully exploit the optimal
control problem structure at hand. Existing structure-exploiting solvers are
fast but they often lack techniques to deal with nonlinearity or rely on
penalty methods to enforce (equality or inequality) path constraints. This
works presents FATROP: a trajectory optimization solver that is fast and
benefits from the salient features of general-purpose nonlinear optimization
solvers. The speed-up is mainly achieved through the use of a specialized
linear solver, based on a Riccati recursion that is generalized to also support
stagewise equality constraints. To demonstrate the algorithm's potential, it is
benchmarked on a set of robot problems that are challenging from a numerical
perspective, including problems with a minimum-time objective and no-collision
constraints. The solver is shown to solve problems for trajectory generation of
a quadrotor, a robot manipulator and a truck-trailer problem in a few tens of
milliseconds. The algorithm's C++-code implementation accompanies this work as
open source software, released under the GNU Lesser General Public License
(LGPL). This software framework may encourage and enable the robotics community
to use trajectory optimization in more challenging applications
Book Review: Feminism & Freedom. by Michael Levin.
Book review: Feminism & Freedom. By Michael Levin. New Brunswick, N.J.: Transaction Books. 1987. Pp. xi, 336. Reviewed by: Brigitte Berger
DYNAMIC BALANCING OF FOUR-BAR LINKAGES: A CONVEX OPTIMIZATION FRAMEWORK FOR EFFICIENTLY OBTAINING GLOBALLY OPTIMAL COUNTERWEIGHTS
This paper focusses on reducing the dynamic reactions (shaking force, shaking moment and driving torque) of plane, crank-rocker four-bars through counterweight addition. Determining the mass parameters of the counterweights constitutes an optimization problem, which is classically considered to be nonlinear and hence difficult to solve. A first contribution of this paper is the proof that this optimization problem can be reformulated as a convex program, that is, a nonlinear optimization problem that still has a unique (and hence guaranteed global) optimum, which can be found with great efficiency. Because of the unique features of this formulation, it becomes possible to investigate (and by the guarantee of obtaining a global optimum, in fact prove) the ultimate limits of dynamic balancing, in a reasonable amount of time. When applied to a particular example, this results in design charts, which clearly illustrate (i) the tradeoff between minimizing the different dynamic reactions, and (ii) the fact that adding counterweights is effective, but at the cost of a significant amount of added mass. These design charts constitute a second contribution of the present work
Subject-exoskeleton contact model calibration leads to accurate interaction force predictions
Knowledge of human–exoskeleton interaction forces is crucial to assess user comfort and effectiveness of the interaction. The subject-exoskeleton collaborative movement and its interaction forces can be predicted in silico using computational modeling techniques. We developed an optimal control framework that consisted of three phases. First, the foot-ground (Phase A) and the subject-exoskeleton (Phase B) contact models were calibrated using three experimental sit-to-stand trials. Then, the collaborative movement and the subject-exoskeleton interaction forces, of six different sit-to-stand trials were predicted (Phase C). The results show that the contact models were able to reproduce experimental kinematics of calibration trials (mean root mean square differences - RMSD - coordinates = 1.1° and velocities = 6.8°/s), ground reaction forces (mean RMSD= 22.9 N), as well as the interaction forces at the pelvis, thigh, and shank (mean RMSD = 5.4 N). Phase C could predict the collaborative movements of prediction trials (mean RMSD coordinates = 3.5° and velocities = 15.0°/s), and their subject-exoskeleton interaction forces (mean RMSD = 13.1° N). In conclusion, this optimal control framework could be used while designing exoskeletons to have in silico knowledge of new optimal movements and their interaction forces.Postprint (author's final draft
Towards Dynamic Visual Servoing for Interaction Control and Moving Targets
International audienceIn this work we present our results on dynamic visual servoing for the case of moving targets while also exploring the possibility of using such a controller for interaction with the environment. We illustrate the derivation of a feature space impedance controller for tracking a moving object as well as an Extended Kalman Filter based on the visual servoing kinematics for increasing the rate of the visual information and estimating the target velocity for both the cases of PBVS and IBVS with image point features. Simulations are carried out to validate the estimator performance during a Peg-in-Hole insertion task with a moving part. Experiments are also conducted on a real redundant manipulator with a low-cost wrist-mounted camera. Details on several implementation issues encountered during implementation are also discussed
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