AMCL: Localization in practice (Tue Oct 22, lecture 14) previous next
Look at the mechanics of SLAM and AMCL

Homework due for today

Legend: : Participation (pass/fail) | : PDF | : Team | : Attachment

  1. Please do Running SLAM on Turtlebot3. Deliverable: create the deliverables as explained in the writeup.

  2. Proposal: Submit your Term Project Proposal. See Final Project Proposals. Deliverable: The proposal as described; team partners can submit the same proposal.

Localization

  • How does the Robot “know” where it is?
  • How does it determine it’s “pose”?
  • Relative to a coordinate system
  • Could be GPS coordinates
  • But more likely coordinate of a given map
Localizing the Robot on a map
  • Key package is called amcl - Adaptive Monte Carlo Localization
  • It’s a deep theoretical area covered in Probabilistic Robotics by Sebsastian Thrun, et al
How it works at a high level
  • Location of robot is its pose
  • Always known with respect to a map coordinate frame (also called sometimes the world coordinate frame)
  • amcl maintains a set of candidate poses plus a probability that they reflect reality
  • As robot moves, actual sensor readings are compared with expected sensor readings for each pose, and the probability of each candidate pose can be updated.
  • Low probability poses are discarded, and high probability poses are updated based on odometry, scan, and probability.
  • For path planning purposes the highest probability pose is used.
  • It is definitely a guess not a certainty.
Simulating localization
  • We will rebuild the map here to make sure things are consistent
  • A new wrinkle here is the use of turtlebot3_simulation which will take the place of teleop
  • That node drives the robot around ranomly while gmapping is building the map
  • After some time has gone by, save the map.
NB Be careful with the filenames of the map. You will get strange errors if the file name given to turtlebot3_navigation is incorrect or not resolvable!
# create the simulated environment called stage_4
$ roslaunch turtlebot3_gazebo turtlebot3_stage_4.launch

# launch a simulated turtlebot3 that drives a random pattern
$ roslaunch turtlebot3_gazebo turtlebot3_simulation.launch

# launch the slam algoritm, which will create an in-memory map data structure
$ roslaunch turtlebot3_slam turtlebot3_slam.launch slam_methods:=gmapping

# After the map looks done, save it into a map file for later.
$ cd ~
$ rosrun map_server map_saver -f stage4

Discuss projects

  • Look at deliverables
  • Discuss status

AMCL in more detail

Monte Carlo
  • That’s where the Casinos are!
  • Algorithms that incorporate random guesses when a direct solution is hard or not feasible
  • Diagram on the board how you would calculate Pi using a Monte Carlo Algorithm
  • And here’s a Video of Monte Carlo Pi
  • Note for PI it’s a very inefficient way to get an accurate result
  • But it illustrates the idea of Monte Carlo estimation
Localization
  • Markov localization
  • Markov means that the new state is dependent only on the previous state (and not the history) and that the probability distributions are
  • Markov localization = state estimation from sensor data
  • Probabilistic algorithm
  • See figure in powerpoints
  • Belief, Sense, Update … etc.
  • But how is the belief represented, and how is the update done efficiently
Particle Filter
  • Instead of “solving” the equation for all data and all points
  • Use a monte-carlo technique
  • Generate a random collection of candidate locations
  • Compute the motion
  • Adjust the probability of each particle

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