A Tutorial on Graph-Based SLAM
Cyrill Stachniss Wolfram Burgard
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A graph
- Node: The poses of the robot at different points
- Edges: Constraints between the poses. Obtained from observations or movement actions
- a probability distribution over a relative transformations between two pose nodes
- eg odometry measurement, aligning observations from two robot locations.
- Include data association problem. included in front-end.
- tasks of pose graph slam
- Graph construction (Front-end)
- Graph optimization (Back-end)
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Solutions for SLAM
Estimating the robot trajectory and the map of environment.
- Filtering: on-line state estimation of current robot position and the map
- Smoothing: estimate the full trajectory from full set of measurements.
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Solving a large error minimization problem to find a configuration of the graph (trajectory) that is maximally consistent with the measurement.
minx∑<i,j>∈CeijTΩijeij \min_{x}{\sum _{<i,j>\in C } e^T_{ij}\Omega _{ij} e_{ij}}\\ xmin<i,j>∈C∑eijTΩijeij
where
eij(xi,xj)=zij−z^ij(xi,xj) e_{ij}(x_i, x_j) = z_{ij}-\hat z_{ij}(x_i, x_j)eij(xi,xj)=zij−z^ij(xi,xj) and Ωij\Omega _{ij} Ωijis information matrix of a virtual measurement.-
Solutions
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Iterative local linearizations: fist order Taylor expansion around the current guess of xxx ( for x is robot pose)
Some question about sparcse construction and sparse Cholesky factorization.
Summary: Gauss-Newton plus Cholesky factorization.
Practical Application: 2D mapping system
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Least Squares on a Manifold (Lie group?) ( for x is robot transfomation)
Define a operator map 3D transformation to manifold.
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Map:
Map can be parametrized as a set of spatially located landmarks, by dense representations like occupancy grids, surface maps, or by raw sensor measurements.
- map is different from 3d reconstruction? (3d surface map is reconstruction?)
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Dynamic Bayesian Network (DBN)
- A directed graph to describe stochastic process
- observed variable nodes (like measurements)
- hidden variable nodes (like robot position state, map)
- state transition model
- observation model

本文详细介绍了图基Simultaneous Localization And Mapping (SLAM)的基本概念,包括图结构中的节点(机器人在不同位置的姿势)、边(来自观测或运动动作的姿势间约束),以及解决SLAM问题的前后端任务。前端负责图构建,后端负责图优化,通过最小化误差来估计机器人轨迹和环境地图。此外,文章还讨论了滤波和平滑两种SLAM解决方案,以及在实际2D地图构建中的应用。
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