Probabilistic Robotics

SLAM and FastSLAM

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SLAM stands for simultaneous localization and mapping originally developed by Hugh Durrant-Whyte and John J. Leonard “Mobile robot localization by tracking geometric beacons” - 1991

The task of building a map while estimating the pose of the robot relative to this map

Why is SLAM hard? Chicken and egg problem: A map is needed to localize the robot and

a pose estimate is needed to build a map Errors correlate!

The SLAM Problem

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SLAM ApplicationsIndoors

Space

Undersea

Underground

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Typical Robot Equipment Mobile robot

Typically want error under 2 cm per meter moved 2° per 45° degrees turned

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Typical Robot Equipment Range finding

sensors Sonar

10 mm accuracy 4 m range

SICK laser 10 mm accuracy 80 m range

IR range finder 1.5 m range

Vision Increasingly viable

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Outline of typical SLAM Process

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Representations Grid maps or scans

[Lu & Milios, 97; Gutmann, 98: Thrun 98; Burgard, 99; Konolige & Gutmann, 00; Thrun, 00; Arras, 99; Haehnel, 01;…]

Landmark-based

[Leonard et al., 98; Castelanos et al., 99: Dissanayake et al., 2001; Montemerlo et al., 2002;…

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Given: The robot’s

controls Observations of

nearby featuresEstimate: Map of features Path of the

robot

The SLAM ProblemA robot moving though an unknown, static environment

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Why is SLAM a hard problem?

SLAM: robot path and map are both unknown!

Robot path error increases errors in the map

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Structure of the Landmark-based SLAM-Problem

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Why is SLAM a hard problem?

In the real world, the mapping between observations and landmarks is unknown

Picking wrong data associations can have catastrophic consequences

Robot poseuncertainty

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Detecting Landmarks Look for spikes in range

data Red dots show table legs

Fit geometric primitives RANSAC line fitting

algorithm Don’t want to fit to all data Pick a random subset Fit a primitive

Classify remaining points as outlier or not Keep if enough points

classify well Keep a full occupancy map

Each cell is a landmark

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(E)KF SLAM Represent robot state and landmark

position all in one big state ( rx, ry, l1x, l1y, l2x, l2y,…) Apply Kalman filter to state update and

observations Uncertainty represented in large

covariance matrix

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For many applications, the time update and measurement equations are NOT linear The KF is not directly applicable

Linearize around the non-linearities Use of the KF is extended to more

realistic situations

Kalman Filter and Linearity

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The Extended Kalman (EKF) is a sub-optimal extension of the original KF algorithm

The EKF allows for estimation of non-linear processes or measurement relationships

The Extended Kalman Filter (EKF)

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Kalman Filter Extended Kalman Filter

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Linearity Assumption Revisited

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Non-linear Function

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EKF Linearization (1)

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EKF Linearization (2)

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EKF Linearization (3)

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Map with N landmarks:(3+2N)-dimensional Gaussian

Can handle hundreds of dimensions

(E)KF-SLAM

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EKF-SLAM

Map Correlation matrix

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EKF-SLAM

Map Correlation matrix

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EKF-SLAM

Map Correlation matrix

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EKF-SLAM Can be quadratic in number of

landmarks Non-linearities a problem

Has led to other approaches

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Is there a dependency between the dimensions of the state space?

If so, can we use the dependency to solve the problem more efficiently?

In the SLAM context The map depends on the poses of the robot. We know how to build a map if the position of the

sensor is known.

Dependencies

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FastSLAM Rao-Blackwellized particle filtering based on

landmarks [Montemerlo et al., 2002] Each landmark is represented by a 2x2

Extended Kalman Filter (EKF) Each particle therefore has to maintain M EKFs

Landmark 1 Landmark 2 Landmark M…x, y,

Landmark 1 Landmark 2 Landmark M…x, y, Particle#1

Landmark 1 Landmark 2 Landmark M…x, y, Particle#2

ParticleN

…

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FastSLAM – Action Update

Particle #1

Particle #2

Particle #3

Landmark #1Filter

Landmark #2Filter

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FastSLAM – Sensor Update

Particle #1

Particle #2

Particle #3

Landmark #1Filter

Landmark #2Filter

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FastSLAM – Sensor Update

Particle #1

Particle #2

Particle #3

Weight = 0.8

Weight = 0.4

Weight = 0.1

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Multi-Hypothesis Data Association

Data association is done on a per-particle basis

Robot pose error is factored out of data association decisions

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Improved Proposal The proposal adapts to the structure

of the environment

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Per-Particle Data Association

Was the observationgenerated by the redor the blue landmark?

P(observation|red) = 0.3 P(observation|blue) = 0.7

Two options for per-particle data association Pick the most probable match Pick an random association weighted by

the observation likelihoods If the probability is too low, generate a new

landmark

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Results – Victoria Park

4 km traverse < 5 m RMS

position error 100 particles

Dataset courtesy of University of Sydney

Blue = GPSYellow = FastSLAM

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Selective Re-sampling Re-sampling is dangerous, since

important samples might get lost(particle depletion problem)

Key question: When should we re-sample?

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Typical Evolution of particle number

visiting new areas closing the

first loop

second loop closure

visiting known areas

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Intel Lab 15 particles four times faster

than real-timeP4, 2.8GHz

5cm resolution during scan matching

1cm resolution in final map

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More Details on FastSLAM M. Montemerlo, S. Thrun, D. Koller, and B. Wegbreit. FastSLAM: A

factored solution to simultaneous localization and mapping, AAAI02

D. Haehnel, W. Burgard, D. Fox, and S. Thrun. An efficient FastSLAM algorithm for generating maps of large-scale cyclic environments from raw laser range measurements, IROS03

M. Montemerlo, S. Thrun, D. Koller, B. Wegbreit. FastSLAM 2.0: An Improved particle filtering algorithm for simultaneous localization and mapping that provably converges. IJCAI-2003

G. Grisetti, C. Stachniss, and W. Burgard. Improving grid-based slam with rao-blackwellized particle filters by adaptive proposals and selective resampling, ICRA05

A. Eliazar and R. Parr. DP-SLAM: Fast, robust simultanous localization and mapping without predetermined landmarks, IJCAI03