Executing slam and navigation: topics functionality

The displays determined by the configuration file rviz_conf.vcg and their descriptions are the following:

1. Floor scan (laser scan): Shows data from a laser scan, with different options for rendering modes, accumulation, etc. sensor_msgs/LaserScan

2. Grid (Grid): Displays a 2D or 3D grid along a plane

3. Obstacles (Grid cells): Displays the obstacles that the navigation stack sees in its costmap. For the robot to avoid collision, the robot footprint should never intersect with a cell that contains an obstacle. nav_msgs/GridCells

4. Inflated obstacles (Grid Cells): Displays obstacles in the navigation stack's costmap inflated by the inscribed radius of the robot. For the robot to avoid collision, the center point of the robot should never overlap with a cell that contains an inflated obstacle. nav_msgs/GridCells

5. Map (Map): Displays a map on the ground plane. nav_msgs/OccupancyGrid

6. Global Plan (Path): Displays the portion of the global plan that the local planner is currently pursuing. nav_msgs/Path

7. Local Plan (Path): Displays the trajectory associated with the velocity commands currently being commanded to the base by the local planner.

8. Current goal (Pose): Displays the goal pose that the navigation stack is attempting to achieve. geometry_msgs/PoseStamped

9. Localisation (Pose Array): Draws a "cloud" of arrows, one for each pose in a pose array. geometry_msgs/PoseArray

10. Robot Footprint (Polygon): Displays the footprint of the robot. geometry_msgs/Polygon

If we open rxgraph we can see that most of the above displays are topics published by the move_base_node and they are subscribing to the rviz node.

If we sk rosnode to display information about the move_base_node it can tell us more clearly which topics it published and to which of them it is subscribing as well as the messages over which they communicate:

Node [/move_base_node]

Publications:
* /move_base_node/local_costmap/obstacles [nav_msgs/GridCells]
* /move_base_node/NavfnROS/NavfnROS_costmap/obstacles [nav_msgs/GridCells]
* /move_base_node/current_goal [geometry_msgs/PoseStamped]
* /move_base_node/global_costmap/unknown_space [nav_msgs/GridCells]
* /move_base_node/local_costmap/inflated_obstacles [nav_msgs/GridCells]
* /move_base_node/NavfnROS/NavfnROS_costmap/robot_footprint [geometry_msgs/PolygonStamped]
* /move_base_node/TrajectoryPlannerROS/global_plan [nav_msgs/Path]
* /move_base_node/local_costmap/voxel_grid [costmap_2d/VoxelGrid]
* /move_base/result [move_base_msgs/MoveBaseActionResult]
* /move_base_node/global_costmap/inflated_obstacles [nav_msgs/GridCells]
* /move_base_node/TrajectoryPlannerROS/local_plan [nav_msgs/Path]
* /move_base/status [actionlib_msgs/GoalStatusArray]
* /cmd_vel [geometry_msgs/Twist]
* /move_base_node/global_costmap/obstacles [nav_msgs/GridCells]
* /rosout [roslib/Log]
* /move_base/goal [move_base_msgs/MoveBaseActionGoal]
* /move_base_node/NavfnROS/plan [nav_msgs/Path]
* /move_base_node/local_costmap/unknown_space [nav_msgs/GridCells]
* /move_base_node/NavfnROS/NavfnROS_costmap/inflated_obstacles [nav_msgs/GridCells]
* /move_base_node/local_costmap/robot_footprint [geometry_msgs/PolygonStamped]
* /move_base/feedback [move_base_msgs/MoveBaseActionFeedback]
* /move_base_node/global_costmap/robot_footprint [geometry_msgs/PolygonStamped]
* /move_base_node/NavfnROS/NavfnROS_costmap/unknown_space [nav_msgs/GridCells]

Subscriptions:
* /time [unknown type]
* /move_base_simple/goal [geometry_msgs/PoseStamped]
* /move_base/goal [move_base_msgs/MoveBaseActionGoal]
* /base_scan [sensor_msgs/LaserScan]
* /map [nav_msgs/OccupancyGrid]
* /tf [tf/tfMessage]
* /clock [roslib/Clock]
* /odom [nav_msgs/Odometry]
* /move_base/cancel [unknown type]
* /reset_time [unknown type]
* /tf_message [unknown type]


This can be done for everyone of the nodes that is running.

Stage and navigation stack tutorial

This entry is based on the following tutorial:
http://www.ros.org/wiki/move_base_stage/Tutorials/stage%20and%20navigation%20stack
Following are the elements of the awesomeros package and a small description of their functions.

1. World file: willow.world

It is located at awesomeros/world/willow.world, it is required by stage and it describes the characteristics of the simulation environment.

2. Map server: willow.yaml

Located at world//willow.yaml. A map is required by navigation stack to work on. The yaml file determines the occupancy, resolution and location characteristics of the map being used.

3. Costmaps: they store information about obstacles in the map

3.1 costmap_common_params.yaml: configurations that both costmaps (local and global) should follow.
3.2 global_costmap_params.yaml: configuration of long-term plans over the entire environment.
3.3 local_costmap_params.yaml: configuration for local planning and obstacle avoidance.

4. Base local planner: base_local_planner_params.yaml

It is responsible for computing velocity commands to send to the mobile base of the robot.

5. Navfn: navfn_params.yaml

It provides a fast interpolated navigation function that can be used to create plans for a mobile base.

6. Move_base: move_base.xml

This package lets you move your robot to the desired position using all the previously mentioned nodes of the navigation stack.

7. Amcl: amcl_node.xml

It is a probabilistic location system for mobile robots in 2D.

8. Rviz: rviz.xml and rviz.vcg.

It is a 3D visualization environment

9. Launch file: launch/robot.launch

It acts as a container for all the elements of the package.

Running the launch file and the rviz configuration file lets us set goals in rviz.

The most significant topics published while running this tutorials are the ones of the move_base node.
And the transforms among frames are broadcasted only by stageros and amcl.

Running the sicklms node and rviz

The sscrovers_laser_pub package is based on the sicktoolbox_wrapper package.

Renato made a modification to the sicklms node which consisted on applying a delay of 22 seconds to it since it is approximately the time that the physical laser takes to power up properly.
I had to modify the ssc_laser.vcg file since with the original one the readings of the laser weren't displayed on rviz. What I did was to check the topics published by and subscribed to the rviz and sicklms nodes.

The changes I made are in lines 5 and 22. Instead of /laser topic I placed /base_scan topic. This worked honestly thanks to a process of trial and error.
In order to run the rviz and sicklms nodes I followed these steps:

1. Run a core

$ roscore

2. Configure the laser port

$ sudo chmod a+rw /dev/ttyUSB1
$ ls -l /dev/ttyUSB1

And something like this should be displayed:

crw-rw-rw- 1 root dialout 166, 0 2009-10-27 14:18 /dev/ttyUSB1

3. Run the sicklms node

$ roscore
$ roscd sscrovers_laser_pub
$ cd bin
$ ./sicklms _port:=/dev/ttyUSB1 _baud:=38400
Connection must be achieved:

 Requesting measured value data stream...
             Data stream started!

4. Start rviz

$ roscd rviz
$ ./bin/rviz -d `rospack find sscrovers_laser_pub`/ssc_laser.vcg


5. Check the communication among nodes through specific topics

$ rxgraph