3,233 research outputs found
Adaptive Sampling For Efficient Online Modelling
This thesis examines methods enabling autonomous systems to make active sampling and planning decisions in real time. Gaussian Process (GP) regression is chosen as a framework for its non-parametric approach allowing flexibility in unknown environments. The first part of the thesis focuses on depth constrained full coverage bathymetric surveys in unknown environments. Algorithms are developed to find and follow a depth contour, modelled with a GP, and produce a depth constrained boundary. An extension to the Boustrophedon Cellular Decomposition, Discrete Monotone Polygonal Partitioning is developed allowing efficient planning for coverage within this boundary. Efficient computational methods such as incremental Cholesky updates are implemented to allow online Hyper Parameter optimisation and fitting of the GP's. This is demonstrated in simulation and the field on a platform built for the purpose. The second part of this thesis focuses on modelling the surface salinity profiles of estuarine tidal fronts. The standard GP model assumes evenly distributed noise, which does not always hold. This can be handled with Heteroscedastic noise. An efficient new method, Parametric Heteroscedastic Gaussian Process regression, is proposed. This is applied to active sample selection on stationary fronts and adaptive planning on moving fronts where a number of information theoretic methods are compared. The use of a mean function is shown to increase the accuracy of predictions whilst reducing optimisation time. These algorithms are validated in simulation. Algorithmic development is focused on efficient methods allowing deployment on platforms with constrained computational resources. Whilst the application of this thesis is Autonomous Surface Vessels, it is hoped the issues discussed and solutions provided have relevance to other applications in robotics and wider fields such as spatial statistics and machine learning in general
Information-Theoretic Stochastic Optimal Control via Incremental Sampling-based Algorithms
This paper considers optimal control of dynamical systems which are
represented by nonlinear stochastic differential equations. It is well-known
that the optimal control policy for this problem can be obtained as a function
of a value function that satisfies a nonlinear partial differential equation,
namely, the Hamilton-Jacobi-Bellman equation. This nonlinear PDE must be solved
backwards in time, and this computation is intractable for large scale systems.
Under certain assumptions, and after applying a logarithmic transformation, an
alternative characterization of the optimal policy can be given in terms of a
path integral. Path Integral (PI) based control methods have recently been
shown to provide elegant solutions to a broad class of stochastic optimal
control problems. One of the implementation challenges with this formalism is
the computation of the expectation of a cost functional over the trajectories
of the unforced dynamics. Computing such expectation over trajectories that are
sampled uniformly may induce numerical instabilities due to the exponentiation
of the cost. Therefore, sampling of low-cost trajectories is essential for the
practical implementation of PI-based methods. In this paper, we use incremental
sampling-based algorithms to sample useful trajectories from the unforced
system dynamics, and make a novel connection between Rapidly-exploring Random
Trees (RRTs) and information-theoretic stochastic optimal control. We show the
results from the numerical implementation of the proposed approach to several
examples.Comment: 18 page
Exploration in Information Distribution Maps
In this paper, a novel solution for autonomous robotic exploration is proposed. The distribution of information in an unknown environment is modeled as an unsteady diffusion process, which can be an appropriate mathematical formulation and analogy for expanding, time-varying, and dynamic environments. This information distribution map is the solution of the diffusion process partial differential equation, and is regressed from sensor data as a Gaussian Process. Optimization of the process parameters leads to an optimal frontier map which describes regions of interest for further exploration. Since the presented approach considers a continuous model of the environment, it can be used to plan smooth exploration paths exploiting the structural dependencies of the environment whilst handling sparse sensors measurements. The performance of the proposed approach is evaluated through simulation results in the well-known Freiburg and Cave maps
A Survey of path following control strategies for UAVs focused on quadrotors
The trajectory control problem, defined as making a vehicle follow a pre-established path in space, can be solved by means of trajectory tracking or path following. In the trajectory tracking problem a timed reference position is tracked. The path following approach removes any time dependence of the problem, resulting in many advantages on the control performance and design. An exhaustive review of path following algorithms applied to quadrotor vehicles has been carried out, the most relevant are studied in this paper. Then, four of these algorithms have been implemented and compared in a quadrotor simulation platform: Backstepping and Feedback Linearisation control-oriented algorithms and NLGL and Carrot-Chasing geometric algorithms.Peer ReviewedPostprint (author's final draft
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