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Why_Sampling.md

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Sampling-based algorithm

Author: Hong Lu

E-mail: [email protected]

[TOC]

What is Motion Planning Problem?

Acknowledgement: This part is completely from MIT16.410 Lecture 15.

Consider a dynamic control system defined as follows:

$$dx/dt = f(x,u), \space x(0) = x_{init}$$ (1)

where $x$ is the state, $u$ is the control.

Given an obstacle set $X_{obs} \subset \it{R}^{d}$, and set a goal set $X_{goal} \subset \it{R}^{d}$, the objective of motion planning problem is to find a control signal $u$ such that the solution of (1) satisfied $x(t) \notin X_{obs}$ for all $t \in \it{R}{+}$, and $X(t) \in X{goal}$ for all $t > T$, for some finite $T \geq 0$. Return failure if no $u$ exists.

THIS IS BASIC PROBLEM IN ROBOTICS BUT VERY HARD TO PROVE

Motion Planning in practice

  • Algebraic planner : explicit representation of environment.

    complete, but impractical;

  • Discretization & graph search (A*, D*, etc.)

    sensitive to graph size and do not scale well to high dimensions;

  • Potential fields / navigation function

    hard to compute.

Sampling-Based algorithms

very successful in practice, based on (batch or incremental) sampling method.

  • Incremental sampling methods are particularly attractive:
    • easy to real-time, on-line implementation
    • applicable to very general dynamic system
    • do not require the explicit enumeration of constraints
    • adaptively multi-resolution methods

Rapidly-exploring Random Trees (RRT)

pros

  • The RRT algorithm is probabilistically complete.

  • The probability goes to 1 exponentially fast, if the environment satisfied certain 'good visibility' conditions.

  • great at finding feasible trajectory.

cons

  • No characterization of quality or cost of the trajectory returned by algorithm.
  • No systematic method for impose temporal / logical constraints.
  • terrible at finding good trajectory.
  • RRT algorithm "traps" itself by disallowing new better path to merge.

and....why?

Rapidly-exploring Random Graph (RRG)

pseudocode

$\bold{V} \leftarrow {q_{start} } \space \bold{E} \leftarrow {}$

for i = 1, 2 , ... , N = $IterationTime$

​ $q_{rand} \leftarrow SamplingFreeSpace​$

​ $q_{nearest} \leftarrow FindNearest(G, q_{rand})$

​ $q_{new }\leftarrow Steer(q_{nearest}, q_{new})​$

​ if $E(q_{nearest}, q_{new}) ​$ is in $FreeSpace​$ then

​ $\bold{X}{near} \leftarrow Near(G, q{new}, MeasurementDist)$

​ endif

endfor

Notion : $RRT^{*}$ is a tree version of $RRG$.

Appendix

what does environment-modelling mean?

from Wikipedia

Environmental modelling is the creation and use of mathematical models of the environment. Environmental modelling may be used purely for research purposes and improved understanding of environmental systems, or for providing an interdisciplinary analysis that can inform decision making and policy.[1]

From my perspective, the whole point is mathematical models while sampling-based algorithm is not needed but it indeed needs map information.

what is online/offline algorithm and which RRT belongs to?

online algorithm is one that can process its input piece-by-piece in a serial fashion, in the order that the input is fed to the algorithm, without having the entire input available from the start.

offline algorithm is given the whole problem data from the beginning and is required to output an answer which solves the problem at hand.

classic(simple) RRT algorithm is not online.

what is Asymptotic optimality? 渐进最优

**Definition : **An algorithm ALG is asymptotic optimality if, for any motion planning problem $P = (x_{free}, x_{init}, x_{goal})$ and cost function $c$ that admit a robust optimal solution with a finite value $c^{*}$，that

$\textrm{P} ({ lim_{i \rightarrow \infin} Y_{i}^{ALG}} = c^{*})​$

**explanation : **This equation means that the probability of algorithm ALG can merge to the optimal value $c^*​$ after $\infin​$ - times planning.

Reference