Action client with feedback in ROS - Mini Project: Build & Apply

Action client with feedback

📖 Scenario: You are building a ROS node that sends a goal to a robot's action server to move to a target position. You want to receive feedback during the movement to monitor progress.

🎯 Goal: Create a ROS action client node that sends a goal to the move_base action server, receives feedback, and prints the feedback progress.

📋 What You'll Learn

Create a ROS action client node named move_client

Send a goal with position x=1.0 and y=2.0

Implement a feedback callback function named feedback_cb

Print feedback progress inside the feedback callback

Use the MoveBaseAction action type from move_base_msgs

💡 Why This Matters

🌍 Real World

Robots often need to move to specific locations while providing progress updates. This project shows how to build a client that commands movement and monitors feedback.

💼 Career

Understanding ROS action clients and feedback handling is essential for robotics software engineers working on autonomous navigation and control.

1

DATA SETUP: Import ROS libraries and create the action client

Import rospy, actionlib, and MoveBaseAction from move_base_msgs.msg. Create an action client named client for move_base using MoveBaseAction.

ROS

import rospy
import actionlib
from move_base_msgs.msg import MoveBaseAction, MoveBaseGoal

# Create the action client named client
# Your code here

Hint

Use actionlib.SimpleActionClient with the server name 'move_base' and action type MoveBaseAction.

2

CONFIGURATION: Wait for the server and create a goal

Call client.wait_for_server() to wait for the action server. Create a MoveBaseGoal object named goal and set its target position to x=1.0 and y=2.0 in the goal.target_pose.pose.position.

ROS

import rospy
import actionlib
from move_base_msgs.msg import MoveBaseAction, MoveBaseGoal

client = actionlib.SimpleActionClient('move_base', MoveBaseAction)

# Wait for the server
# Create a MoveBaseGoal named goal
# Set goal.target_pose.pose.position.x = 1.0
# Set goal.target_pose.pose.position.y = 2.0
# Your code here

Hint

Remember to set the header.frame_id to 'map' and orientation w to 1.0 for a valid pose.

3

CORE LOGIC: Define the feedback callback and send the goal

Define a function named feedback_cb that takes one argument feedback. Inside it, print the feedback's current position feedback.base_position.pose.position.x and feedback.base_position.pose.position.y. Then send the goal using client.send_goal(goal, feedback_cb=feedback_cb).

ROS

import rospy
import actionlib
from move_base_msgs.msg import MoveBaseAction, MoveBaseGoal

client = actionlib.SimpleActionClient('move_base', MoveBaseAction)
client.wait_for_server()
goal = MoveBaseGoal()
goal.target_pose.header.frame_id = 'map'
goal.target_pose.header.stamp = rospy.Time.now()
goal.target_pose.pose.position.x = 1.0
goal.target_pose.pose.position.y = 2.0
goal.target_pose.pose.orientation.w = 1.0

# Define feedback_cb(feedback) to print feedback base_position x and y
# Send the goal with feedback_cb
# Your code here

Hint

The feedback callback receives a feedback object with base_position. Use an f-string to print the x and y positions.

4

COMPLETION: Wait for the result and shutdown the node

Call client.wait_for_result() to wait for the action to finish. Then call rospy.signal_shutdown('Goal reached') to cleanly stop the node.

ROS

import rospy
import actionlib
from move_base_msgs.msg import MoveBaseAction, MoveBaseGoal

def feedback_cb(feedback):
    print(f"Feedback: x={feedback.base_position.pose.position.x}, y={feedback.base_position.pose.position.y}")

client = actionlib.SimpleActionClient('move_base', MoveBaseAction)
client.wait_for_server()
goal = MoveBaseGoal()
goal.target_pose.header.frame_id = 'map'
goal.target_pose.header.stamp = rospy.Time.now()
goal.target_pose.pose.position.x = 1.0
goal.target_pose.pose.position.y = 2.0
goal.target_pose.pose.orientation.w = 1.0

client.send_goal(goal, feedback_cb=feedback_cb)

# Wait for the result
# Shutdown the node with message 'Goal reached'
# Your code here

Hint

Use client.wait_for_result() to block until the goal finishes, then call rospy.signal_shutdown() with the message.

Practice

1. What is the main purpose of an action client in ROS when using feedback?

easy

A. To directly control robot hardware without feedback

B. To send goals to an action server and receive progress updates

C. To replace service calls with simpler requests

D. To only receive the final result without intermediate updates

Solution

1. Step 1: Understand the role of an action client

   An action client sends goals to an action server and can receive feedback during execution.

2. Step 2: Identify feedback purpose

   Feedback callbacks provide real-time progress updates while the task runs.

3. Final Answer:

   To send goals to an action server and receive progress updates -> Option B

4. Quick Check:

   Action client = send goals + get feedback [OK]

Hint: Action clients send goals and get feedback updates [OK]

Common Mistakes:

• Thinking action clients only get final results
• Confusing action clients with service clients
• Ignoring feedback callbacks

2. Which of the following is the correct way to create an action client with feedback in ROS Python?

easy

A. client = actionlib.SimpleActionClient('move', MoveAction, feedback_cb=feedback_cb)

B. client = actionlib.ActionClient('move', MoveAction, feedback_cb=feedback_cb)

C. client = actionlib.SimpleActionClient('move', MoveAction)

D. client = actionlib.ActionClient('move', MoveAction)

Solution

1. Step 1: Recall ROS Python action client syntax

   The standard way is to use SimpleActionClient with server name and action type.

2. Step 2: Feedback callback is set separately

   Feedback callbacks are registered when sending goals, not in constructor.

3. Final Answer:

   client = actionlib.SimpleActionClient('move', MoveAction) -> Option C

4. Quick Check:

   SimpleActionClient + feedback_cb separate [OK]

Hint: Feedback callback is passed when sending goal, not client creation [OK]

Common Mistakes:

• Passing feedback_cb in constructor (wrong)
• Using ActionClient instead of SimpleActionClient
• Missing action type argument

3. Given this ROS Python snippet, what will be printed if the feedback callback receives progress=50?

def feedback_cb(feedback):
    print(f"Progress: {feedback.progress}%")

client = actionlib.SimpleActionClient('task', TaskAction)
client.wait_for_server()
goal = TaskGoal()
client.send_goal(goal, feedback_cb=feedback_cb)

medium

A. Progress: 50%

B. Progress: feedback.progress%

C. Progress: None%

D. No output because feedback_cb is not called

Solution

1. Step 1: Understand feedback callback behavior

   The feedback_cb prints the progress attribute from feedback with formatting.

2. Step 2: Substitute progress value

   If feedback.progress is 50, the print outputs "Progress: 50%" exactly.

3. Final Answer:

   Progress: 50% -> Option A

4. Quick Check:

   Formatted string with feedback.progress = output [OK]

Hint: Feedback callback prints formatted progress value [OK]

Common Mistakes:

• Printing variable name instead of value
• Assuming feedback_cb is not called
• Ignoring string formatting

4. What is wrong with this ROS Python action client code snippet?

client = actionlib.SimpleActionClient('task', TaskAction)
client.send_goal(TaskGoal(), feedback_cb=feedback_cb)
client.wait_for_server()

medium

A. wait_for_server() should be called before send_goal()

B. feedback_cb cannot be passed to send_goal()

C. TaskGoal() must be assigned to a variable before sending

D. SimpleActionClient requires a third argument for feedback

Solution

1. Step 1: Check action client usage order

   wait_for_server() must be called before sending a goal to ensure server is ready.

2. Step 2: Identify error in code order

   Here, send_goal() is called before wait_for_server(), which is incorrect.

3. Final Answer:

   wait_for_server() should be called before send_goal() -> Option A

4. Quick Check:

   wait_for_server() before send_goal() [OK]

Hint: Always wait for server before sending goal [OK]

Common Mistakes:

• Calling send_goal before wait_for_server
• Misunderstanding feedback_cb usage
• Assuming feedback_cb is constructor argument

5. You want to implement an action client that sends multiple goals sequentially and prints feedback for each. Which approach correctly handles feedback for each goal without mixing updates?

hard

A. Ignore feedback and only wait for final results of each goal

B. Use a single global feedback callback for all goals without distinguishing them

C. Send all goals at once without waiting and use one feedback callback

D. Create a new feedback callback function for each goal that captures its goal ID

Solution

1. Step 1: Understand feedback handling for multiple goals

   Each goal's feedback should be handled separately to avoid mixing progress updates.

2. Step 2: Use closures or separate callbacks

   Creating a unique feedback callback per goal that captures its ID ensures correct feedback association.

3. Final Answer:

   Create a new feedback callback function for each goal that captures its goal ID -> Option D

4. Quick Check:

   Separate feedback callbacks per goal prevent mix-up [OK]

Hint: Use unique feedback callbacks per goal to track progress separately [OK]

Common Mistakes:

• Using one callback for all goals causing mixed feedback
• Sending all goals simultaneously without control
• Ignoring feedback defeats purpose of action client