Logistics

• Projects

Review

1. Consists of a grid of “costs” (costmap) and a coordinate system
2. Map server “publishes” the map on the /map topic

SLAM

1. An algorithm that builds up a map while collecting sensor and analyzing it statistically
2. The map as it is being built is published by the map_server
3. You can save the map to a file so that it remembers it to the next run

Navigation

1. AMCL: Given lidar sensor data and a map, estimates the true location of the robot
2. Calculation is reset when amcl receives a new pose_estimate
3. This estimate is different from /odom (why?)
4. AMCL publishes a /tf transform between /map and /odom (note this is the “highest probability” location)

move_base

• A sophisticated, pluggable architecture
• Allows for new algorithms to be designed and tested
• Parameterized for configuration
• Uses ROS “actions” to initiate, monitor, and report success
• Initial goal is: /MoveBaseActionGoal
• 5: Actions

Components of move_base

• Global Planner: Creates a plan from the current position (AMCL) to the goal position with respect to a map. May use any of a number of algorithms (including A*)
• Local Planner: Given the local map (built in real time from sensors), considers location and velocity and computes the near part of the plan.
• costmap-2d: General package to build, maintain, update and access a multi-layer 2d map. Used for both local and global maps and for other things
• Recovery behaviors: either map clearing or rotation recovery heuristics used when robot appears to be stuck.

FLows

How move_base works - global planner

1. Uses base_global planner to comnpute a path from current position to goal
2. This plan is with respect to the global costmap (from /map topic and map_server)
3. Look at ROS global planner for visuals
4. Interesting parameters:

/use_dijkstra (bool, default: true) # If true, use dijkstra's algorithm. Otherwise, A*.
/use_quadratic (bool, default: true) # If true, use the quadratic approximation of the potential. Otherwise, use a simpler calculation.
/use_grid_path (bool, default: false) # If true, create a path that follows the grid boundaries. Otherwise, use a gradient descent method.

How move_base works - local planner

1. Create a local map grid from the local costmap) with costs representing distances to the final goal
2. Uses base_local_planner to determine the next motion in the immediate future

The two book chapters

• 10 part2: Navigating
• 10 part3: Move Base

Topics for further study

• The SLAM Toolbox
• The Python Robotics Pacakge

Thank you. Questions?  (random Image from picsum.photos)