Setting navigation goals in ROS - Mini Project: Build & Apply

Setting navigation goals

📖 Scenario: You are programming a robot to move to specific points in a room. You want to tell the robot exactly where to go by setting navigation goals.

🎯 Goal: Build a simple ROS node that sets a navigation goal with exact coordinates and orientation for the robot to reach.

📋 What You'll Learn

Create a ROS node named goal_sender

Define a navigation goal with position x=2.0, y=3.0 and orientation w=1.0

Use the move_base_msgs/MoveBaseGoal message to set the goal

Send the goal to the move_base action server

💡 Why This Matters

🌍 Real World

Robots in warehouses or homes need to move to specific locations safely and accurately. Setting navigation goals lets you control where the robot goes.

💼 Career

Understanding how to send navigation goals is essential for robotics engineers working with autonomous robots and ROS.

1

Create the ROS node and import necessary libraries

Write the first lines of a Python ROS node named goal_sender. Import rospy, actionlib, and MoveBaseAction and MoveBaseGoal from move_base_msgs.msg. Initialize the node with rospy.init_node('goal_sender').

ROS

# Your code here to import rospy, actionlib, MoveBaseAction, MoveBaseGoal and initialize node

Hint

Use import statements for the libraries and rospy.init_node to start the node.

2

Create an action client to communicate with move_base

Create an action client named client using actionlib.SimpleActionClient for the move_base action server with MoveBaseAction. Wait for the server to be ready using client.wait_for_server().

ROS

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

rospy.init_node('goal_sender')
# Create action client and wait for server

Hint

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

3

Define the navigation goal with position and orientation

Create a MoveBaseGoal object named goal. Set goal.target_pose.header.frame_id to "map". Set goal.target_pose.header.stamp to rospy.Time.now(). Set goal.target_pose.pose.position.x to 2.0 and goal.target_pose.pose.position.y to 3.0. Set goal.target_pose.pose.orientation.w to 1.0.

ROS

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

rospy.init_node('goal_sender')
client = actionlib.SimpleActionClient('move_base', MoveBaseAction)
client.wait_for_server()
# Define the navigation goal with position and orientation

Hint

Use the MoveBaseGoal message fields to set the position and orientation inside target_pose.pose. Don't forget to set the header timestamp.

4

Send the goal to the move_base action server

Send the goal to the move_base server using client.send_goal(goal). Then wait for the result with client.wait_for_result().

ROS

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

rospy.init_node('goal_sender')
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 = 2.0
goal.target_pose.pose.position.y = 3.0
goal.target_pose.pose.orientation.w = 1.0
# Send the goal and wait for the result

Hint

Use send_goal to send the goal and wait_for_result to wait until the robot reaches the goal.

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