Static transforms for fixed frames in ROS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to publish a static transform broadcaster in ROS.

ROS

static_broadcaster = tf2_ros.[1]()

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

AStaticTransformBroadcaster

BTransformListener

CBuffer

DTransformBroadcaster

Attempts:

3 left

💡 Hint

Common Mistakes

Using TransformBroadcaster instead of StaticTransformBroadcaster

Confusing Buffer or TransformListener with broadcaster classes

The StaticTransformBroadcaster is used to publish static transforms in ROS.

2fill in blank

medium

Complete the code to create a TransformStamped message and set its header frame_id.

ROS

transform = geometry_msgs.msg.TransformStamped()
transform.header.[1] = 'world'

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

Astamp

Bchild_frame_id

Cseq

Dframe_id

Attempts:

3 left

💡 Hint

Common Mistakes

Setting child_frame_id in the header instead of frame_id

Using stamp or seq incorrectly

The frame_id in the header specifies the parent frame of the transform.

3fill in blank

hard

Fix the error in setting the translation of the transform.

ROS

transform.transform.translation.x = [1]
transform.transform.translation.y = 0.0
transform.transform.translation.z = 0.0

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

ATrue

B'1.0'

C1.0

DNone

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string '1.0' instead of a float 1.0

Assigning None or boolean values

The translation values must be floats, not strings or other types.

4fill in blank

hard

Fill both blanks to set the rotation as a quaternion representing no rotation.

ROS

transform.transform.rotation.[1] = 0.0
transform.transform.rotation.[2] = 1.0

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

Ax

By

Cw

Dz

Attempts:

3 left

💡 Hint

Common Mistakes

Setting w to 0.0 or x to 1.0 incorrectly

Mixing up quaternion components

For no rotation, quaternion x, y, z are 0.0 and w is 1.0.

5fill in blank

hard

Fill all three blanks to publish the static transform with the current time and child frame 'camera'.

ROS

transform.header.stamp = [1]
transform.child_frame_id = [2]
static_broadcaster.[3]([transform])

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

Arospy.Time.now()

B'camera'

CsendTransform

Drospy.get_time()

Attempts:

3 left

💡 Hint

Common Mistakes

Using rospy.get_time() which returns float, not ROS time

Forgetting to set child_frame_id

Calling wrong method to publish

Use rospy.Time.now() for current time, set child frame to 'camera', and call sendTransform to publish.

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