Static transforms for fixed frames in ROS - Mini Project: Build & Apply

Static transforms for fixed frames

📖 Scenario: You are working on a robot that has a fixed sensor mounted on its base. To help the robot understand where the sensor is relative to its base, you need to create a static transform. This transform will not change over time because the sensor is fixed in place.

🎯 Goal: Create a ROS static transform publisher node that broadcasts the fixed position and orientation of the sensor frame relative to the robot base frame.

📋 What You'll Learn

Create a ROS node that publishes a static transform

Use the static_transform_publisher from tf2_ros

Set the translation to x=0.5, y=0.0, z=0.2

Set the rotation as a quaternion x=0.0, y=0.0, z=0.0, w=1.0

Set the parent frame as base_link and child frame as sensor_link

💡 Why This Matters

🌍 Real World

Robots often have sensors or parts fixed in place relative to their base. Static transforms help the robot understand where these parts are in space.

💼 Career

Knowing how to publish static transforms is essential for robot developers working with ROS to integrate sensors and coordinate frames correctly.

1

Create the ROS node script

Create a Python script named static_tf_publisher.py and import rclpy and StaticTransformBroadcaster from tf2_ros. Initialize the ROS node with the name static_tf_publisher.

ROS

# Your code here to import rclpy and StaticTransformBroadcaster, and initialize the node

Hint

Use import rclpy and from tf2_ros.static_transform_broadcaster import StaticTransformBroadcaster. Then call rclpy.init() and create a node with Node('static_tf_publisher').

2

Create the static transform message

Create a geometry_msgs.msg.TransformStamped message named static_transformStamped. Set its header.frame_id to base_link and child_frame_id to sensor_link. Set the translation to x=0.5, y=0.0, z=0.2 and rotation quaternion to x=0.0, y=0.0, z=0.0, w=1.0.

ROS

import geometry_msgs.msg

# Your code here to create static_transformStamped and set frame ids, translation, and rotation

Hint

Import geometry_msgs.msg. Create static_transformStamped as geometry_msgs.msg.TransformStamped(). Set header.frame_id and child_frame_id. Then set translation and rotation values.

3

Publish the static transform

Create a StaticTransformBroadcaster instance named broadcaster using the node. Use broadcaster.sendTransform() to send a list containing static_transformStamped.

ROS

# Your code here to create StaticTransformBroadcaster and send the transform

Hint

Create broadcaster as StaticTransformBroadcaster(node). Then call broadcaster.sendTransform() with a list containing static_transformStamped.

4

Keep the node alive

Add rclpy.spin(node) to keep the node running and then call rclpy.shutdown() after spinning.

ROS

# Your code here to spin the node and shutdown

Hint

Use rclpy.spin(node) to keep the node alive and rclpy.shutdown() to clean up after.

Practice

1. What is the main purpose of using static_transform_publisher in ROS?

easy

A. To dynamically update the position of a robot part during movement

B. To define a fixed position and orientation between two frames that do not move relative to each other

C. To visualize sensor data in RViz

D. To launch multiple ROS nodes simultaneously

Solution

1. Step 1: Understand the role of static transforms

   Static transforms are used to represent fixed relationships between frames that do not change over time.

2. Step 2: Identify the function of static_transform_publisher

   This command publishes a fixed transform between two frames, meaning the position and orientation remain constant.

3. Final Answer:

   To define a fixed position and orientation between two frames that do not move relative to each other -> Option B

4. Quick Check:

   Static transform = fixed frame relation [OK]

Hint: Static transforms fix frame relations that never change [OK]

Common Mistakes:

• Confusing static with dynamic transforms
• Thinking it updates during robot movement
• Mixing it up with sensor data visualization

2. Which of the following is the correct syntax to publish a static transform from frame base_link to camera_link with translation (1, 0, 0) and no rotation using static_transform_publisher?

easy

A. static_transform_publisher base_link camera_link 1 0 0 0 0 0 1 100

B. static_transform_publisher 0 0 0 1 0 0 0 base_link camera_link 100

C. static_transform_publisher 1 0 0 0 0 0 1 base_link camera_link 100

D. static_transform_publisher 1 0 0 0 0 0 0 base_link camera_link 100

Solution

1. Step 1: Recall the static_transform_publisher argument order

   The syntax is: static_transform_publisher x y z qx qy qz qw frame_id child_frame_id period_in_ms.

2. Step 2: Match values to the syntax

   Translation is (1, 0, 0), rotation quaternion is (0, 0, 0, 1) for no rotation, frames are base_link and camera_link, and period is 100 ms.

3. Final Answer:

   static_transform_publisher 1 0 0 0 0 0 1 base_link camera_link 100 -> Option C

4. Quick Check:

   Correct argument order and values = static_transform_publisher 1 0 0 0 0 0 1 base_link camera_link 100 [OK]

Hint: Remember translation then quaternion then frames [OK]

Common Mistakes:

• Swapping frame order
• Incorrect quaternion values for no rotation
• Placing frames before numbers

3. Given the command:
static_transform_publisher 0 0 1 0 0 0 1 world map 50
What does this static transform represent?

medium

A. A translation of 1 meter along the Y-axis from world to map with 180 degrees rotation

B. A translation of 1 meter along the X-axis from map to world with no rotation

C. A rotation of 1 radian around the Z-axis between world and map

D. A translation of 1 meter along the Z-axis from world to map with no rotation

Solution

1. Step 1: Analyze translation and rotation values

   Translation is (0, 0, 1), meaning 1 meter along Z-axis. Quaternion (0, 0, 0, 1) means no rotation.

2. Step 2: Identify frame order

   The transform is from world frame to map frame, so map is positioned 1 meter above world.

3. Final Answer:

   A translation of 1 meter along the Z-axis from world to map with no rotation -> Option D

4. Quick Check:

   Translation Z=1, no rotation, world to map [OK]

Hint: Check translation vector and quaternion carefully [OK]

Common Mistakes:

• Mixing up frame order
• Misreading quaternion as rotation angle
• Confusing axis directions

4. You run the command:
static_transform_publisher 0 0 0 0 0 0 0 base_link camera_link 100
but no transform appears in tf. What is the likely problem?

medium

A. The quaternion rotation is invalid because the w component is zero

B. The translation values are all zero, so no transform is published

C. The frame names are reversed; camera_link should be first

D. The period 100 is too short to publish the transform

Solution

1. Step 1: Check quaternion validity

   A quaternion must be normalized; here (0,0,0,0) is invalid. The w component cannot be zero for a valid rotation.

2. Step 2: Understand effect on transform publishing

   An invalid quaternion causes the transform publisher to fail silently, so no transform appears in tf.

3. Final Answer:

   The quaternion rotation is invalid because the w component is zero -> Option A

4. Quick Check:

   Quaternion w=0 invalid = no transform [OK]

Hint: Quaternion w must not be zero for valid rotation [OK]

Common Mistakes:

• Assuming zero translation means no transform
• Swapping frame order without checking syntax
• Thinking publish period affects visibility immediately

5. You want to create a static transform tree where base_link is fixed to odom with translation (0, 0, 0) and no rotation, and camera_link is fixed to base_link with translation (0.5, 0, 1) and a 90-degree rotation around the Y-axis. Which two commands correctly publish these static transforms?

hard

A. static_transform_publisher 0 0 0 0 0 0 1 odom base_link 100
static_transform_publisher 0.5 0 1 0 0.7071 0 0.7071 base_link camera_link 100

B. static_transform_publisher 0 0 0 0 0 0 1 base_link odom 100
static_transform_publisher 0.5 0 1 0 0.7071 0 0.7071 camera_link base_link 100

C. static_transform_publisher 0 0 0 0 0 0 1 odom base_link 100
static_transform_publisher 0.5 0 1 0 1 0 0 base_link camera_link 100

D. static_transform_publisher 0 0 0 0 0 0 0 odom base_link 100
static_transform_publisher 0.5 0 1 0 0.7071 0 0.7071 base_link camera_link 100

Solution

1. Step 1: Verify first transform from odom to base_link

   Translation is zero, no rotation quaternion is (0,0,0,1), frames ordered correctly as parent then child.

2. Step 2: Verify second transform from base_link to camera_link

   Translation is (0.5,0,1). A 90-degree rotation around Y-axis quaternion is approximately (0, 0.7071, 0, 0.7071). Frames ordered correctly.

3. Final Answer:

   static_transform_publisher 0 0 0 0 0 0 1 odom base_link 100 static_transform_publisher 0.5 0 1 0 0.7071 0 0.7071 base_link camera_link 100 -> Option A

4. Quick Check:

   Correct quaternions and frame order = static_transform_publisher 0 0 0 0 0 0 1 odom base_link 100 static_transform_publisher 0.5 0 1 0 0.7071 0 0.7071 base_link camera_link 100 [OK]

Hint: Check quaternion for 90° Y rotation and frame order carefully [OK]

Common Mistakes:

• Swapping parent and child frames
• Using wrong quaternion for rotation
• Setting quaternion w to zero