194 research outputs found
Real-Time Planning with Multi-Fidelity Models for Agile Flights in Unknown Environments
Autonomous navigation through unknown environments is a challenging task that
entails real-time localization, perception, planning, and control. UAVs with
this capability have begun to emerge in the literature with advances in
lightweight sensing and computing. Although the planning methodologies vary
from platform to platform, many algorithms adopt a hierarchical planning
architecture where a slow, low-fidelity global planner guides a fast,
high-fidelity local planner. However, in unknown environments, this approach
can lead to erratic or unstable behavior due to the interaction between the
global planner, whose solution is changing constantly, and the local planner; a
consequence of not capturing higher-order dynamics in the global plan. This
work proposes a planning framework in which multi-fidelity models are used to
reduce the discrepancy between the local and global planner. Our approach uses
high-, medium-, and low-fidelity models to compose a path that captures
higher-order dynamics while remaining computationally tractable. In addition,
we address the interaction between a fast planner and a slower mapper by
considering the sensor data not yet fused into the map during the collision
check. This novel mapping and planning framework for agile flights is validated
in simulation and hardware experiments, showing replanning times of 5-40 ms in
cluttered environments.Comment: ICRA 201
Recent Advances in Path Integral Control for Trajectory Optimization: An Overview in Theoretical and Algorithmic Perspectives
This paper presents a tutorial overview of path integral (PI) control
approaches for stochastic optimal control and trajectory optimization. We
concisely summarize the theoretical development of path integral control to
compute a solution for stochastic optimal control and provide algorithmic
descriptions of the cross-entropy (CE) method, an open-loop controller using
the receding horizon scheme known as the model predictive path integral (MPPI),
and a parameterized state feedback controller based on the path integral
control theory. We discuss policy search methods based on path integral
control, efficient and stable sampling strategies, extensions to multi-agent
decision-making, and MPPI for the trajectory optimization on manifolds. For
tutorial demonstrations, some PI-based controllers are implemented in MATLAB
and ROS2/Gazebo simulations for trajectory optimization. The simulation
frameworks and source codes are publicly available at
https://github.com/INHA-Autonomous-Systems-Laboratory-ASL/An-Overview-on-Recent-Advances-in-Path-Integral-Control.Comment: 16 pages, 9 figure
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
Contributions to Open Problems on Cable Driven Robots and Persistent Manifolds for the Synthesis of Mechanisms
Although many efforts are continuously devoted to the advancement of robotics, there are still many open and unresolved problems to be faced. This thesis, therefore, sets out to tackle some of them with the aim of scratching the surface and look a little further for new ideas or solutions.
The topics covered are mainly two. The first part deals with the development and improvement of control techniques for cable-driven robots. The second focuses on the study of persistent manifolds seen as constituting aspects of theoretical kinematics.
In detail,
-Part I deals with cable-driven platforms. In it, both techniques for selecting cable tensions and the design of a robust controller are developed. The aim is, therefore, to enhance the two building blocks of the overall control scheme in order to improve the performance of these robots during the execution of tracking tasks.
-- The first chapter introduces to open problems and recalls the main concepts necessary to understand the following chapters;
-- the contribution of the second chapter consists of the introduction of the Analytic Centre. It allows the generation of continuous and differentiable tension profiles while taking into account non-linear phenomena such as friction in the computation of tensions to be applied;
-- the third chapter, although still at a preliminary stage, introduces sensitivity for tension calculation methods, offering perspectives of considerable interest for tension control in the current scientific context;
-- the fourth chapter proposes the design of an adaptive controller. It allows external disturbances and/or uncertainties in the model to be faced such that the task can be performed with as little error as possible. The controller architecture is the innovative peculiarity conferring autonomy to cable systems. Initially applied to counteract wind in aerial systems it is now also used for cable breakage scenarios;
-- the conclusions, at first, draw together the results obtained. In addition, they emphasise the lack of the techniques introduced in order to outline possible future paths and topics that need further investigation.
- Part II delves into theoretical kinematics. The discovery and classification of invariant screw systems shed light on numerous aspects of robot mobility and synthesis. Nevertheless, this generated the emergence of new ideas and questions that are still unresolved. Among them, one of the more notable concerns the identification and classification of 5-dimensional persistent manifolds.
-- Similarly to the first part, the first chapter provides an overview of the problems addressed and the theoretical notions necessary to understand the subsequent contributions;
-- the second chapter contributes by directly tackling the above-mentioned question by exploiting the properties of dual quaternions, the Study quadric and differential geometry. A library of 5-persistent varieties, so far missing in the literature, is presented along with theorems that complete and generalise previous ones in the literature;
-- an original work, concerning line motions and synthesis of mechanisms that generate them, is reported in the third chapter as a spin-off of the studies on persistent manifolds;
-- the conclusions wrap up the obtained results trying to highlight gaps and deficiencies to be dealt with in the future. Here, two small sections are dedicated to ongoing works regarding the persistence definition and the screw systems' invariants and subvariants
Geometric Control of two Quadrotors Carrying a Rigid Rod with Elastic Cables
This paper presents the design of a geometric trajectory tracking controller
for the cooperative task of two quadrotor UAVs (unmanned aerial vehicles)
carrying and transporting a rigid bar, which is attached to the quadrotors via
inflexible elastic cables. The elasticity of the cables together with
techniques of singular perturbation allows a reduction in the model to that of
a similar model with inelastic cables. In this reduced model, we design a
controller such that the rod exponentially tracks a given desired trajectory
for its position and attitude, under some assumptions on initial error. We then
show that exponential tracking in the reduced model corresponds to exponential
tracking of the original elastic model. We also show that the previously
defined control scheme provides uniform ultimate boundedness in the presence of
unstructured bounded disturbances.Comment: 20 pages, 3 figure
NEPTUNE: Non-Entangling Planning for Multiple Tethered Unmanned Vehicles
Despite recent progress on trajectory planning of multiple robots and path
planning of a single tethered robot, planning of multiple tethered robots to
reach their individual targets without entanglements remains a challenging
problem. In this paper, we present a complete approach to address this problem.
Firstly, we propose a multi-robot tether-aware representation of homotopy,
using which we can efficiently evaluate the feasibility and safety of a
potential path in terms of (1) the cable length required to reach a target
following the path, and (2) the risk of entanglements with the cables of other
robots. Then, the proposed representation is applied in a decentralized and
online planning framework that includes a graph-based kinodynamic trajectory
finder and an optimization-based trajectory refinement, to generate
entanglement-free, collision-free and dynamically feasible trajectories. The
efficiency of the proposed homotopy representation is compared against existing
single and multiple tethered robot planning approaches. Simulations with up to
8 UAVs show the effectiveness of the approach in entanglement prevention and
its real-time capabilities. Flight experiments using 3 tethered UAVs verify the
practicality of the presented approach.Comment: Accepted for publication in IEEE Transaction on Robotic
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