Exercises#

This page contains three take-home exercises that reinforce the concepts from Lecture 12. Each exercise asks you to write code from scratch based on a specification – no starter code is provided.

All files should be created inside your ~/enpm605_ws/src/ workspace in the appropriate packages (namespace_demo, remapping_demo, or bt_demo).

Exercise 1 – Multi-Camera Namespace Launch

Goal

Write a launch file that starts three instances of the same camera node, each in a different namespace, and verify they are fully isolated.

Specification

Create a launch file at namespace_demo/launch/three_cameras.launch.py that:

  1. Starts three camera_demo_exe nodes under namespaces /front, /left, and /right.

  2. Each node must use output="screen" and emulate_tty=True.

Verification:

ros2 launch namespace_demo three_cameras.launch.py

In a second terminal:

ros2 node list
# Expected: /front/camera_node, /left/camera_node, /right/camera_node

ros2 topic list
# Expected: /front/image_raw, /left/image_raw, /right/image_raw

rqt_graph
# Three isolated publisher nodes, no cross-talk
Exercise 2 – Add a New Condition Node to the BT

Goal

Add a MaxDistanceTraveled condition node to the behavior tree that stops the robot after it has traveled a configurable maximum distance.

Specification

Create bt_demo/bt_demo/max_distance.py that implements:

  1. A MaxDistanceTraveled class inheriting from py_trees.behaviour.Behaviour.

  2. Constructor takes name and max_distance (float, meters).

  3. In setup(), subscribe to /odometry/filtered.

  4. Track the cumulative distance traveled by computing the Euclidean distance between consecutive odometry readings.

  5. In update():

    • Return SUCCESS if the total distance is below max_distance (robot can keep going).

    • Return FAILURE if the total distance exceeds max_distance (robot should stop).

  6. Add this condition to the root Sequence in main_drive_to_goal.py, before the GoalNotReached? condition:

Sequence (DriveToGoal, memory=False)
|-- MaxDistanceTraveled?    [NEW]
|-- GoalNotReached?
|-- Selector (DriveOrRecover, memory=True)
    |-- ...

Verification:

ros2 launch bt_demo drive_to_goal.py goal_x:=100.0 max_distance:=3.0

The robot should stop after approximately 3 meters, even though the goal is 100 meters away.

Exercise 3 – Topic Remapping with Multiple Subscribers

Goal

Create a launch file that connects two camera publishers to a single image processor subscriber using topic remapping.

Specification

Create remapping_demo/launch/multi_remap.launch.py that:

  1. Starts two camera_demo_exe nodes with names front_camera and rear_camera.

  2. Remaps their topics:

    • front_camera: image_raw -> /sensors/front/image

    • rear_camera: image_raw -> /sensors/rear/image

  3. Starts two image_processor_exe nodes with names front_processor and rear_processor.

  4. Remaps their subscriptions:

    • front_processor: camera/image -> /sensors/front/image

    • rear_processor: camera/image -> /sensors/rear/image

Verification:

ros2 launch remapping_demo multi_remap.launch.py

In a second terminal:

ros2 topic list
# Expected: /sensors/front/image and /sensors/rear/image

rqt_graph
# front_camera -> /sensors/front/image -> front_processor
# rear_camera  -> /sensors/rear/image  -> rear_processor

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