Preempting actions in ROS - Mini Project: Build & Apply

Preempting Actions in ROS

📖 Scenario: You are building a robot control system using ROS (Robot Operating System). Sometimes the robot needs to stop its current task and start a new one immediately. This is called preempting an action.We will create a simple ROS action client that sends a goal, then preempts it by sending a cancel request.

🎯 Goal: Build a ROS action client node that sends a goal to an action server, then preempts (cancels) the goal after a short delay.

📋 What You'll Learn

Create a ROS action client node in Python

Send a goal to the action server named /example_action

Preempt the goal by sending a cancel request

Use the SimpleActionClient from actionlib

Use the ExampleAction action definition

💡 Why This Matters

🌍 Real World

Robots often need to stop their current task immediately when new commands arrive. Preempting actions allows smooth task switching.

💼 Career

Understanding ROS action clients and preemption is essential for robotics software engineers working on autonomous systems.

1

Create the ROS action client node and import necessary modules

Write the first 5 lines of a Python ROS node script that imports rospy, actionlib, and the ExampleAction and ExampleGoal messages from the example_action_msgs.msg package. Also initialize the ROS node with the name example_action_client.

ROS

# Your code here

Hint

Use import statements for modules and rospy.init_node() to start the node.

2

Create the SimpleActionClient and wait for the server

Create a variable called client that is a SimpleActionClient for the /example_action server using ExampleAction. Then call client.wait_for_server() to wait until the server is ready.

ROS

import rospy
import actionlib
from example_action_msgs.msg import ExampleAction, ExampleGoal

rospy.init_node('example_action_client')
# Create the SimpleActionClient and wait for the server
# Your code here

Hint

Use actionlib.SimpleActionClient with the server name and action type, then call wait_for_server().

3

Send a goal to the action server

Create a variable called goal as an instance of ExampleGoal. Then send this goal to the server using client.send_goal(goal).

ROS

import rospy
import actionlib
from example_action_msgs.msg import ExampleAction, ExampleGoal

rospy.init_node('example_action_client')

client = actionlib.SimpleActionClient('/example_action', ExampleAction)
client.wait_for_server()
# Create a goal and send it
# Your code here

Hint

Create the goal object and use send_goal() to send it.

4

Preempt the goal by canceling it

After sending the goal, call client.cancel_goal() to preempt (cancel) the current goal.

ROS

import rospy
import actionlib
from example_action_msgs.msg import ExampleAction, ExampleGoal

rospy.init_node('example_action_client')

client = actionlib.SimpleActionClient('/example_action', ExampleAction)
client.wait_for_server()

goal = ExampleGoal()
client.send_goal(goal)
# Cancel the goal to preempt it
# Your code here

Hint

Use cancel_goal() on the client to stop the current 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