Setting navigation goals in ROS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a navigation goal is sent with an unreachable position?

Consider a ROS node sending a navigation goal to a robot using the move_base action. The goal position is outside the known map boundaries. What will the robot's navigation stack most likely do?

ROS

action_client.send_goal(goal_pose);

A The robot immediately reports failure and cancels the goal.

B The robot teleports to the goal position without path planning.

C The robot ignores the goal and continues its current task without any feedback.

D The robot attempts to navigate but eventually times out and reports failure.

💡 Hint

Think about how the navigation stack handles unreachable goals and timeouts.

When a goal is unreachable, the navigation stack tries to find a path. If it fails within a timeout, it reports failure. It does not teleport or ignore the goal.

❓ state_output

intermediate

What is the state of the action client after sending a valid navigation goal?

After sending a valid navigation goal using the ROS action client, what is the expected state of the client before the robot reaches the goal?

ROS

action_client.send_goal(goal_pose);
current_state = action_client.get_state();

A PREEMPTED

B ACTIVE

C SUCCEEDED

D LOST

💡 Hint

Consider the action client states during goal execution.

After sending a goal, the action client state is ACTIVE while the robot is moving towards the goal.

📝 Syntax

advanced

Which code snippet correctly sets a navigation goal using ROS Python API?

Select the code snippet that correctly creates and sends a navigation goal to move_base using ROS Python.

A

goal = MoveBaseGoal()
goal.target_pose.header.frame_id = 'map'
goal.target_pose.pose.position.x = 1.0
client.send_goal(goal)

B

goal = MoveBaseGoal()
goal.pose.position.x = 1.0
client.send_goal(goal)

C

goal = MoveBaseGoal()
goal.target_pose.position.x = 1.0
client.send_goal(goal)

D

goal = MoveBaseGoal()
goal.target_pose.header.frame = 'map'
goal.target_pose.pose.position.x = 1.0
client.send_goal(goal)

💡 Hint

Check the correct attribute names for setting the frame and pose.

The correct attributes are target_pose.header.frame_id and target_pose.pose.position.x. Option A uses these correctly.

🔧 Debug

advanced

Why does the robot never reach the navigation goal?

A ROS navigation node sends a goal to move_base, but the robot never reaches it and the goal status stays ACTIVE indefinitely. What is the most likely cause?

ROS

goal.target_pose.header.frame_id = 'map'
goal.target_pose.pose.position.x = 5.0
goal.target_pose.pose.position.y = 5.0
client.send_goal(goal)

A The robot's localization is lost, so it cannot plan a path.

B The move_base node is not running, so the goal is ignored.

C The goal frame_id is incorrect and does not match the robot's map frame.

D The goal position is inside an obstacle, so the robot cannot move.

💡 Hint

Think about what happens if the robot does not know where it is on the map.

If localization fails, the robot cannot plan a path and will keep the goal ACTIVE without progress.

🧠 Conceptual

expert

What is the effect of setting the 'planner_frequency' parameter too low in ROS navigation?

In the ROS navigation stack, the 'planner_frequency' parameter controls how often the global planner runs. What happens if this frequency is set too low while the robot is moving towards a goal?

A The robot will plan paths more frequently, causing high CPU usage but better responsiveness.

B The robot will ignore the global planner and rely only on the local planner.

C The robot may follow outdated paths and react slowly to changes in the environment.

D The robot will stop moving until a new plan is generated.

💡 Hint

Consider how often the robot updates its path to avoid obstacles.

A low planner_frequency means the global planner updates paths less often, so the robot may not adapt quickly to new obstacles or changes.

Practice

1. What is the main purpose of setting a navigation goal in ROS?

easy

A. To tell the robot where to move using coordinates and orientation

B. To change the robot's color

C. To update the robot's software version

D. To stop the robot immediately

Solution

1. Step 1: Understand navigation goals

   Navigation goals specify a target position and orientation for the robot to reach.

2. Step 2: Identify the purpose

   Setting a navigation goal directs the robot where to move in its environment.

3. Final Answer:

   To tell the robot where to move using coordinates and orientation -> Option A

4. Quick Check:

   Navigation goal = target position [OK]

Hint: Navigation goals set robot's target position and direction [OK]

Common Mistakes:

• Confusing navigation goals with software updates
• Thinking navigation goals change robot appearance
• Assuming navigation goals stop the robot

2. Which ROS frame is commonly used to set navigation goals relative to the environment?

easy

A. "base_link" frame

B. "odom" frame

C. "map" frame

D. "camera" frame

Solution

1. Step 1: Identify common frames in ROS navigation

   Frames like "base_link", "odom", and "map" represent different coordinate systems.

2. Step 2: Choose the frame for environment-relative goals

   The "map" frame is fixed to the environment and used for global navigation goals.

3. Final Answer:

   "map" frame -> Option C

4. Quick Check:

   Environment-relative goals use "map" frame [OK]

Hint: "map" frame is fixed to environment, perfect for goals [OK]

Common Mistakes:

• Using "base_link" which is robot-centered
• Choosing "odom" which drifts over time
• Selecting "camera" which is unrelated to navigation

3. Given this ROS Python snippet to send a navigation goal:

goal.target_pose.header.frame_id = "map"
goal.target_pose.pose.position.x = 2.0
goal.target_pose.pose.position.y = 3.0
goal.target_pose.pose.orientation.w = 1.0
client.send_goal(goal)
client.wait_for_result()

What will the robot do?

medium

A. Throw an error due to missing orientation x, y, z

B. Move to position (3.0, 2.0) on the map facing backward

C. Stay still because orientation is incomplete

D. Move to position (2.0, 3.0) on the map facing forward

Solution

1. Step 1: Analyze position and orientation values

   Position is set to x=2.0, y=3.0; orientation.w=1.0 means no rotation (facing forward).

2. Step 2: Understand client commands

   Sending goal and waiting for result commands the robot to move to the specified pose.

3. Final Answer:

   Move to position (2.0, 3.0) on the map facing forward -> Option D

4. Quick Check:

   Goal pose = (2.0,3.0), orientation forward [OK]

Hint: Orientation w=1 means facing forward, position sets location [OK]

Common Mistakes:

• Mixing up x and y coordinates
• Assuming missing orientation x,y,z causes error
• Thinking orientation.w=1 means backward

4. What is wrong with this ROS Python code snippet for setting a navigation goal?

goal.target_pose.header.frame_id = "map"
goal.target_pose.pose.position.x = 1.0
goal.target_pose.pose.position.y = 2.0
client.send_goal(goal)
client.wait_for_result()

medium

A. frame_id should be "odom" instead of "map"

B. Missing orientation in goal.pose causes navigation failure

C. Position values must be integers, not floats

D. client.wait_for_result() must come before send_goal()

Solution

1. Step 1: Check completeness of goal pose

   Orientation is not set, which is required for a valid navigation goal.

2. Step 2: Verify other parts

   frame_id "map" is correct; position floats are allowed; wait_for_result() must follow send_goal().

3. Final Answer:

   Missing orientation in goal.pose causes navigation failure -> Option B

4. Quick Check:

   Orientation required for goal [OK]

Hint: Always set orientation when sending navigation goals [OK]

Common Mistakes:

• Ignoring missing orientation
• Changing frame_id unnecessarily
• Misordering wait_for_result and send_goal

5. You want the robot to navigate to two different points sequentially using ROS move_base. Which approach correctly sets and sends these goals?

hard

A. Send first goal, wait for result, then send second goal and wait again

B. Send both goals at the same time without waiting

C. Send second goal first, then send first goal after completion

D. Only send the last goal; the robot will visit both automatically

Solution

1. Step 1: Understand sequential goal sending

   To visit points in order, send one goal and wait for it to finish before sending the next.

2. Step 2: Evaluate options

   Sending both simultaneously confuses the robot; sending second first breaks order; last goal alone ignores first.

3. Final Answer:

   Send first goal, wait for result, then send second goal and wait again -> Option A

4. Quick Check:

   Sequential goals need wait between sends [OK]

Hint: Wait for each goal to finish before sending next [OK]

Common Mistakes:

• Sending multiple goals simultaneously
• Assuming robot queues goals automatically
• Sending goals in wrong order