Preempting actions in ROS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create an action client in ROS.

ROS

client = actionlib.SimpleActionClient('[1]', MoveBaseAction)

Drag options to blanks, or click blank then click option'

A/scan

B/cmd_vel

C/move_base

D/odom

Attempts:

3 left

💡 Hint

Common Mistakes

Using a topic name instead of an action server name.

Confusing command velocity topic with action server.

The action client must connect to the correct action server name, which is /move_base for navigation actions.

2fill in blank

medium

Complete the code to send a goal to the action server.

ROS

client.[1](goal)

Drag options to blanks, or click blank then click option'

Asend_goal

Bcancel_goal

Cwait_for_result

Dget_result

Attempts:

3 left

💡 Hint

Common Mistakes

Using wait_for_result instead of send_goal.

Trying to get result before sending goal.

To send a goal to the action server, use the send_goal method.

3fill in blank

hard

Fix the error in the code to preempt the current goal.

ROS

client.[1]()

Drag options to blanks, or click blank then click option'

Asend_goal

Bwait_for_result

Cget_result

Dcancel_goal

Attempts:

3 left

💡 Hint

Common Mistakes

Using send_goal instead of cancel_goal.

Trying to get result instead of canceling.

To preempt or cancel the current goal, use the cancel_goal method.

4fill in blank

hard

Fill both blanks to check if the action server is ready and then send a goal.

ROS

if client.[1]():
    client.[2](goal)

Drag options to blanks, or click blank then click option'

Await_for_server

Bsend_goal

Ccancel_goal

Dget_result

Attempts:

3 left

💡 Hint

Common Mistakes

Sending goal before server is ready.

Using wrong methods for waiting or sending.

First, wait for the action server to be ready using wait_for_server. Then send the goal with send_goal.

5fill in blank

hard

Fill all three blanks to cancel the current goal, wait for the server, and send a new goal.

ROS

client.[1]()
if client.[2]():
    client.[3](goal)

Drag options to blanks, or click blank then click option'

Acancel_goal

Bwait_for_server

Csend_goal

Dget_result

Attempts:

3 left

💡 Hint

Common Mistakes

Not canceling the old goal before sending a new one.

Not waiting for the server before sending.

First, cancel the current goal with cancel_goal. Then wait for the server using wait_for_server. Finally, send the new goal with send_goal.

Practice

1.

What does cancel_goal() do in ROS action clients?

easy

A. It requests to stop the currently running action goal.

B. It starts a new action goal immediately.

C. It pauses the action without stopping it.

D. It resets the action server to its initial state.

Solution

1. Step 1: Understand the purpose of cancel_goal()

   This function is used to request the cancellation of the current goal being processed by the action server.

2. Step 2: Differentiate from other actions

   Starting a new goal or pausing are different operations; cancel_goal() specifically requests stopping the current goal.

3. Final Answer:

   It requests to stop the currently running action goal. -> Option A

4. Quick Check:

   cancel_goal() stops current goal [OK]

Hint: Remember: cancel_goal() means stop current action [OK]

Common Mistakes:

• Confusing cancel_goal() with starting a new goal
• Thinking cancel_goal() pauses instead of cancels
• Assuming cancel_goal() resets the server

2.

Which of the following is the correct sequence to preempt an action in ROS?

1. cancel_goal()
2. send_goal(new_goal)

easy

A. Call cancel_goal() first, then send_goal(new_goal).

B. Call send_goal(new_goal) first, then cancel_goal().

C. Call cancel_goal() twice, then send_goal(new_goal).

D. Only call send_goal(new_goal) without canceling.

Solution

1. Step 1: Understand preempting order

   You must first cancel the current goal to stop it before sending a new goal to replace it.

2. Step 2: Confirm correct method calls

   Calling cancel_goal() first ensures the old task stops, then send_goal(new_goal) starts the new task.

3. Final Answer:

   Call cancel_goal() first, then send_goal(new_goal). -> Option A

4. Quick Check:

   Cancel before sending new goal [OK]

Hint: Cancel first, then send new goal to preempt [OK]

Common Mistakes:

• Sending new goal before canceling old one
• Calling cancel_goal() multiple times unnecessarily
• Skipping cancel_goal() and sending new goal directly

3.

Given this ROS action client code snippet, what will be the output behavior?

client.send_goal(goal1)
client.cancel_goal()
client.send_goal(goal2)

What happens to goal1 and goal2?

medium

A. Neither goal runs because cancel_goal() stops all actions.

B. goal1 runs fully; goal2 is ignored.

C. Both goal1 and goal2 run simultaneously.

D. goal1 is canceled; goal2 starts running.

Solution

1. Step 1: Analyze the cancel_goal() effect

   After sending goal1, cancel_goal() requests to stop it immediately.

2. Step 2: Understand sending goal2

   Sending goal2 after canceling goal1 starts the new action goal.

3. Final Answer:

   goal1 is canceled; goal2 starts running. -> Option D

4. Quick Check:

   Cancel old goal, start new goal [OK]

Hint: Cancel stops old goal; new send_goal starts new task [OK]

Common Mistakes:

• Assuming goal1 runs fully despite cancel
• Thinking both goals run at once
• Believing cancel_goal() stops all future goals

4.

Identify the error in this ROS action client code snippet:

client.send_goal(goal1)
client.send_goal(goal2)
client.cancel_goal()

What is wrong with this sequence?

medium

A. The code should call cancel_goal() twice to cancel both goals.

B. You cannot send two goals without canceling the first one first.

C. Calling cancel_goal() after sending two goals does not preempt correctly.

D. There is no error; this sequence is correct.

Solution

1. Step 1: Understand goal handling

   Sending two goals in a row without canceling means the second goal overrides the first immediately.

2. Step 2: Analyze cancel_goal() timing

   Calling cancel_goal() after both sends only cancels the currently active goal, not preempting properly between goals.

3. Final Answer:

   Calling cancel_goal() after sending two goals does not preempt correctly. -> Option C

4. Quick Check:

   Cancel before sending new goal to preempt [OK]

Hint: Cancel before sending next goal, not after [OK]

Common Mistakes:

• Sending multiple goals without canceling first
• Assuming cancel_goal() cancels all previous goals
• Calling cancel_goal() too late to preempt

5.

You want to preempt a running action only if it has been active for more than 5 seconds. Which approach correctly implements this logic?

if client.get_state() == ACTIVE and client.get_time_active() > 5:
    client.cancel_goal()
    client.send_goal(new_goal)

hard

A. You must call cancel_goal() before checking the state.

B. This approach correctly preempts only after 5 seconds of activity.

C. You should send the new goal first, then cancel the old one.

D. ROS actions do not support checking active time for preemption.

Solution

1. Step 1: Check action state and duration

   The code checks if the action is active and has been running longer than 5 seconds.

2. Step 2: Cancel and send new goal conditionally

   Only if both conditions are true, it cancels the current goal and sends a new one, correctly preempting.

3. Final Answer:

   This approach correctly preempts only after 5 seconds of activity. -> Option B

4. Quick Check:

   Check state and time before canceling [OK]

Hint: Check active state and time before canceling [OK]

Common Mistakes:

• Canceling without checking active time
• Sending new goal before canceling old one
• Assuming ROS lacks timing info for actions