2,492 research outputs found
Dynamic interpolation for obstacle avoidance on Riemannian manifolds
This work is devoted to studying dynamic interpolation for obstacle
avoidance. This is a problem that consists of minimizing a suitable energy
functional among a set of admissible curves subject to some interpolation
conditions. The given energy functional depends on velocity, covariant
acceleration and on artificial potential functions used for avoiding obstacles.
We derive first-order necessary conditions for optimality in the proposed
problem; that is, given interpolation and boundary conditions we find the set
of differential equations describing the evolution of a curve that satisfies
the prescribed boundary values, interpolates the given points and is an
extremal for the energy functional.
We study the problem in different settings including a general one on a
Riemannian manifold and a more specific one on a Lie group endowed with a
left-invariant metric. We also consider a sub-Riemannian problem. We illustrate
the results with examples of rigid bodies, both planar and spatial, and
underactuated vehicles including a unicycle and an underactuated unmanned
vehicle.Comment: Comments welcom
Fast, Safe, and Propellant-Efficient Spacecraft Planning under Clohessy-Wiltshire-Hill Dynamics
This paper presents a sampling-based motion planning algorithm for real-time
and propellant-optimized autonomous spacecraft trajectory generation in
near-circular orbits. Specifically, this paper leverages recent algorithmic
advances in the field of robot motion planning to the problem of
impulsively-actuated, propellant-optimized rendezvous and proximity operations
under the Clohessy-Wiltshire-Hill (CWH) dynamics model. The approach calls upon
a modified version of the Fast Marching Tree (FMT*) algorithm to grow a set of
feasible trajectories over a deterministic, low-dispersion set of sample points
covering the free state space. To enforce safety, the tree is only grown over
the subset of actively-safe samples, from which there exists a feasible
one-burn collision avoidance maneuver that can safely circularize the
spacecraft orbit along its coasting arc under a given set of potential thruster
failures. Key features of the proposed algorithm include: (i) theoretical
guarantees in terms of trajectory safety and performance, (ii) amenability to
real-time implementation, and (iii) generality, in the sense that a large class
of constraints can be handled directly. As a result, the proposed algorithm
offers the potential for widespread application, ranging from on-orbit
satellite servicing to orbital debris removal and autonomous inspection
missions.Comment: Submitted to the AIAA Journal of Guidance, Control, and Dynamics
(JGCD) special issue entitled "Computational Guidance and Control". This
submission is the journal version corresponding to the conference manuscript
"Real-Time, Propellant-Efficient Spacecraft Planning under
Clohessy-Wiltshire-Hill Dynamics" accepted to the 2016 IEEE Aerospace
Conference in Big Sky, MT, US
Provably Safe and Robust Drone Routing via Sequential Path Planning: A Case Study in San Francisco and the Bay Area
Provably safe and scalable multi-vehicle path planning is an important and
urgent problem due to the expected increase of automation in civilian airspace
in the near future. Hamilton-Jacobi (HJ) reachability is an ideal tool for
analyzing such safety-critical systems and has been successfully applied to
several small-scale problems. However, a direct application of HJ reachability
to large scale systems is often intractable because of its
exponentially-scaling computation complexity with respect to system dimension,
also known as the "curse of dimensionality". To overcome this problem, the
sequential path planning (SPP) method, which assigns strict priorities to
vehicles, was previously proposed; SPP allows multi-vehicle path planning to be
done with a linearly-scaling computation complexity. In this work, we
demonstrate the potential of SPP algorithm for large-scale systems. In
particular, we simulate large-scale multi-vehicle systems in two different
urban environments, a city environment and a multi-city environment, and use
the SPP algorithm for trajectory planning. SPP is able to efficiently design
collision-free trajectories in both environments despite the presence of
disturbances in vehicles' dynamics. To ensure a safe transition of vehicles to
their destinations, our method automatically allocates space-time reservations
to vehicles while accounting for the magnitude of disturbances such as wind in
a provably safe way. Our simulation results show an intuitive multi-lane
structure in airspace, where the number of lanes and the distance between the
lanes depend on the size of disturbances and other problem parameters.Comment: Submitted to AIAA Journal of Guidance, Control, and Dynamics. arXiv
admin note: substantial text overlap with arXiv:1611.0836
Online Sampling in the Parameter Space of a Neural Network for GPU-accelerated Motion Planning of Autonomous Vehicles
This paper proposes online sampling in the parameter space of a neural
network for GPU-accelerated motion planning of autonomous vehicles. Neural
networks are used as controller parametrization since they can handle nonlinear
non-convex systems and their complexity does not scale with prediction horizon
length. Network parametrizations are sampled at each sampling time and then
held constant throughout the prediction horizon. Controls still vary over the
prediction horizon due to varying feature vectors fed to the network.
Full-dimensional vehicles are modeled by polytopes. Under the assumption of
obstacle point data, and their extrapolation over a prediction horizon under
constant velocity assumption, collision avoidance reduces to linear inequality
checks. Steering and longitudinal acceleration controls are determined
simultaneously. The proposed method is designed for parallelization and
therefore well-suited to benefit from continuing advancements in hardware such
as GPUs. Characteristics of proposed method are illustrated in 5 numerical
simulation experiments including dynamic obstacle avoidance, waypoint tracking
requiring alternating forward and reverse driving with maximal steering, and a
reverse parking scenario.Comment: 8 pages, 8 figures, 3 tables, conference pape
A Distributed Model Predictive Control Framework for Road-Following Formation Control of Car-like Vehicles (Extended Version)
This work presents a novel framework for the formation control of multiple
autonomous ground vehicles in an on-road environment. Unique challenges of this
problem lie in 1) the design of collision avoidance strategies with obstacles
and with other vehicles in a highly structured environment, 2) dynamic
reconfiguration of the formation to handle different task specifications. In
this paper, we design a local MPC-based tracking controller for each individual
vehicle to follow a reference trajectory while satisfying various constraints
(kinematics and dynamics, collision avoidance, \textit{etc.}). The reference
trajectory of a vehicle is computed from its leader's trajectory, based on a
pre-defined formation tree. We use logic rules to organize the collision
avoidance behaviors of member vehicles. Moreover, we propose a methodology to
safely reconfigure the formation on-the-fly. The proposed framework has been
validated using high-fidelity simulations.Comment: Extended version of the conference paper submission on ICARCV'1
Formal Verification of Obstacle Avoidance and Navigation of Ground Robots
The safety of mobile robots in dynamic environments is predicated on making
sure that they do not collide with obstacles. In support of such safety
arguments, we analyze and formally verify a series of increasingly powerful
safety properties of controllers for avoiding both stationary and moving
obstacles: (i) static safety, which ensures that no collisions can happen with
stationary obstacles, (ii) passive safety, which ensures that no collisions can
happen with stationary or moving obstacles while the robot moves, (iii) the
stronger passive friendly safety in which the robot further maintains
sufficient maneuvering distance for obstacles to avoid collision as well, and
(iv) passive orientation safety, which allows for imperfect sensor coverage of
the robot, i. e., the robot is aware that not everything in its environment
will be visible. We complement these provably correct safety properties with
liveness properties: we prove that provably safe motion is flexible enough to
let the robot still navigate waypoints and pass intersections. We use hybrid
system models and theorem proving techniques that describe and formally verify
the robot's discrete control decisions along with its continuous, physical
motion. Moreover, we formally prove that safety can still be guaranteed despite
sensor uncertainty and actuator perturbation, and when control choices for more
aggressive maneuvers are introduced. Our verification results are generic in
the sense that they are not limited to the particular choices of one specific
control algorithm but identify conditions that make them simultaneously apply
to a broad class of control algorithms
Funnel Libraries for Real-Time Robust Feedback Motion Planning
We consider the problem of generating motion plans for a robot that are
guaranteed to succeed despite uncertainty in the environment, parametric model
uncertainty, and disturbances. Furthermore, we consider scenarios where these
plans must be generated in real-time, because constraints such as obstacles in
the environment may not be known until they are perceived (with a noisy sensor)
at runtime. Our approach is to pre-compute a library of "funnels" along
different maneuvers of the system that the state is guaranteed to remain within
(despite bounded disturbances) when the feedback controller corresponding to
the maneuver is executed. We leverage powerful computational machinery from
convex optimization (sums-of-squares programming in particular) to compute
these funnels. The resulting funnel library is then used to sequentially
compose motion plans at runtime while ensuring the safety of the robot. A major
advantage of the work presented here is that by explicitly taking into account
the effect of uncertainty, the robot can evaluate motion plans based on how
vulnerable they are to disturbances.
We demonstrate and validate our method using extensive hardware experiments
on a small fixed-wing airplane avoiding obstacles at high speed (~12 mph),
along with thorough simulation experiments of ground vehicle and quadrotor
models navigating through cluttered environments. To our knowledge, these
demonstrations constitute one of the first examples of provably safe and robust
control for robotic systems with complex nonlinear dynamics that need to plan
in real-time in environments with complex geometric constraints.Comment: International Journal of Robotics Research (To Appear
Reach-Avoid Problems with Time-Varying Dynamics, Targets and Constraints
We consider a reach-avoid differential game, in which one of the players aims
to steer the system into a target set without violating a set of state
constraints, while the other player tries to prevent the first from succeeding;
the system dynamics, target set, and state constraints may all be time-varying.
The analysis of this problem plays an important role in collision avoidance,
motion planning and aircraft control, among other applications. Previous
methods for computing the guaranteed winning initial conditions and strategies
for each player have either required augmenting the state vector to include
time, or have been limited to problems with either no state constraints or
entirely static targets, constraints and dynamics. To incorporate time-varying
dynamics, targets and constraints without the need for state augmentation, we
propose a modified Hamilton-Jacobi-Isaacs equation in the form of a
double-obstacle variational inequality, and prove that the zero sublevel set of
its viscosity solution characterizes the capture basin for the target under the
state constraints. Through this formulation, our method can compute the capture
basin and winning strategies for time-varying games at no additional
computational cost with respect to the time-invariant case. We provide an
implementation of this method based on well-known numerical schemes and show
its convergence through a simple example; we include a second example in which
our method substantially outperforms the state augmentation approach.Comment: Submitted to 18th International Conference on Hybrid Systems:
Computation and Control (HSCC 2015) v2: Corrected typos and introduced minor
esthetic changes (equation 11 moved to the top of page 4); conclusion
slightly shortened for concisenes
On the Construction of Safe Controllable Regions for Affine Systems with Applications to Robotics
This paper studies the problem of constructing in-block controllable (IBC)
regions for affine systems. That is, we are concerned with constructing regions
in the state space of affine systems such that all the states in the interior
of the region are mutually accessible through the region's interior by applying
uniformly bounded inputs. We first show that existing results for checking
in-block controllability on given polytopic regions cannot be easily extended
to address the question of constructing IBC regions. We then explore the
geometry of the problem to provide a computationally efficient algorithm for
constructing IBC regions. We also prove the soundness of the algorithm. We then
use the proposed algorithm to construct safe speed profiles for different
robotic systems, including fully-actuated robots, ground robots modeled as
unicycles with acceleration limits, and unmanned aerial vehicles (UAVs).
Finally, we present several experimental results on UAVs to verify the
effectiveness of the proposed algorithm. For instance, we use the proposed
algorithm for real-time collision avoidance for UAVs.Comment: 17 pages, 18 figures, under review for publication in Automatic
Distributed Obstacle and Multi-Robot Collision Avoidance in Uncertain Environments
This paper tackles the distributed leader-follower (L-F) control problem for
heterogeneous mobile robots in unknown environments requiring obstacle
avoidance, inter-robot collision avoidance, and reliable robot communications.
To prevent an inter-robot collision, we employ a virtual propulsive force
between robots. For obstacle avoidance, we present a novel distributed
Negative-Imaginary (NI) variant formation tracking control approach and a
dynamic network topology methodology which allows the formation to change its
shape and the robot to switch their roles. In the case of communication or
sensor loss, a UAV, controlled by a Strictly-Negative-Imaginary (SNI)
controller with good wind resistance characteristics, is utilized to track the
position of the UGV formation using its camera. Simulations and indoor
experiments have been conducted to validate the proposed methods
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