ar_maze

Package ar_maze aimed to provide robot navigation through the maze with ar tags. In the beginning robot is given by a list of ar tags. Reaching them one by one, robot successfully completes the maze.

Package consists of:

• nav_tags node
• nav_if_walls node
• ar_maze.launch launch file
  However, for package functioning there are two default packages required: ar_track_alvar and robotont_laserscan_to_distance.

Nodes

The main logic in algorithm that is enclosed in nodes nav_tags and nav_if_walls is looking for and approaching the ar tags one by one to complete the maze. In case of not finding the tag, robot starts rotation and motion avoiding obstacles in order to find the (next) ar tag.

NAV_TAGS node

nav_tags node subscribes to the topic "/ar_pose_marker" and starts receiving information about ar tags. When it receives the tag with right id, it publishes robot's velocity under the topic "/cmd_vel" to approach this tag. If there is no ar tag present or it is present, but with wrong index, robot starts rotation. When completed one full rotation and still did not find the tag, nav_tags node set the parameter "ar_present" as False.
The tasks for nav_tags node can be characterized as:

• identifying the ar tag with correct id for navigation while receiving messages of all found ar tags
• setting up the robot linear and angular velocity that depends from position and orientation of correct ar tag found in order to approach it
• setting up the parameter "ar_present" to describe the state of the algorithm (robot found (navigates) ar tag/robot can`t find ar tag)

NAV_IF_WALLS node

nav_if_walls node subscribes to the "/scan_to_distance" topic. Once it receives that not "ar_present", it publishes messages under the topic "/cmd_vel" in such a way: robot does one full rotation and then moving forward a bit. If there is an obstacle robot rotates while not avoid it and continue motion. This appears constantly while not receiving parameter "ar_present" as True from the nav_tags node.
The tasks for nav_if_walls node are:

• checking for the "ar_present" parameter to be False
• setting up the robot angular and linear velocity to complete one full rotation and straight line motion
• completing the previous task only if nearest obstacle is far enough
• if there is an obstacle either on the left, right or center side, setting up the robot angular velocity while not avoid it

Communication between nodes proceeds via ROS parameter "ar_present" that is set up in nav_tags node.

Algorithm in action is shown here.

These nodes can be used also separately. nav_if_walls node can be used for simple motion with obstacle avoidance. nav_tags can be used for simple finding and approaching ar tags as shown here.

Launch file

Launch file ar_maze.launch includes:

• list of ar tags' ids setted up as parameter
• individualMarkersNoKinect node from the ar_track_alvar package with appropriate parameters
• distance_from_depth_image launch file from the robotont_laserscan_to_distance package
• nav_tags node with ar tags list parameter
• nav_if_walls node
  

How to run the algorithm on Robotont robot

1.Build the package and source the catkin workspace:

catkin build
source devel/setup.bash

2.Modify the list with ar tags for maze navigation in 8th line of ar_maze.launch (/launch/ar_maze.launch)

3.Launch the ar_maze.launch file:

roslaunch ar_maze ar_maze.launch