Interactive markers for teleoperation in ROS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize an interactive marker server.

ROS

server = interactive_markers.InteractiveMarkerServer([1])

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

A"my_marker_server"

B42

CTrue

DNone

Attempts:

3 left

💡 Hint

Common Mistakes

Using a number instead of a string for the server name.

Passing None or a boolean value instead of a string.

The InteractiveMarkerServer requires a name string to identify the server instance.

2fill in blank

medium

Complete the code to create a 6-DOF interactive marker control.

ROS

control = InteractiveMarkerControl()
control.name = "move_6dof"
control.interaction_mode = InteractiveMarkerControl.[1]

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

ABUTTON

BMOVE_ROTATE_3D

CMENU

DNONE

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing MENU mode which is for context menus.

Choosing BUTTON mode which is for simple clicks.

MOVE_ROTATE_3D mode allows 6 degrees of freedom control for teleoperation.

3fill in blank

hard

Fix the error in the code to insert the interactive marker into the server.

ROS

server.[1](int_marker)

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

Aadd

Bappend

Cinsert

Dpush

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'add' which does not exist on the server.

Using 'append' which is a list method, not server method.

The correct method to add an interactive marker to the server is 'insert'.

4fill in blank

hard

Fill both blanks to set the orientation mode and update the server.

ROS

control.orientation_mode = InteractiveMarkerControl.[1]
server.[2]()

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

AFIXED

BapplyChanges

CVIEW_FACING

Dcommit

Attempts:

3 left

💡 Hint

Common Mistakes

Using FIXED orientation mode which does not face the user.

Using commit() which is not a server method.

Orientation mode VIEW_FACING makes the control face the user. applyChanges() updates the server.

5fill in blank

hard

Fill all three blanks to create a pose, assign it to the marker, and insert it.

ROS

pose = geometry_msgs.Pose()
pose.position.x = [1]
pose.orientation.w = [2]
int_marker.pose = pose
server.[3](int_marker)

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

A0.5

B1.0

Cinsert

D0.0

Attempts:

3 left

💡 Hint

Common Mistakes

Setting orientation w to 0.0 which is invalid for a quaternion.

Using a wrong method name instead of insert.

Position x is set to 0.5, orientation w to 1.0 for no rotation, and insert() adds the marker to the server.

Practice

1. What is the main purpose of interactive markers in ROS teleoperation?

easy

A. To visually control robots by moving markers in 3D space

B. To write robot control code without any user input

C. To display static images of the robot status

D. To log robot sensor data for offline analysis

Solution

1. Step 1: Understand interactive markers role

   Interactive markers allow users to move and rotate markers in a 3D view to control robots easily.

2. Step 2: Compare options with purpose

   Only To visually control robots by moving markers in 3D space describes visual control via markers; others describe unrelated tasks.

3. Final Answer:

   To visually control robots by moving markers in 3D space -> Option A

4. Quick Check:

   Interactive markers = Visual robot control [OK]

Hint: Interactive markers let you move robot parts visually [OK]

Common Mistakes:

• Thinking markers are only for displaying data
• Confusing teleoperation with offline logging
• Assuming no user input is needed

2. Which ROS message type is commonly used to create an interactive marker for teleoperation?

easy

A. std_msgs/String

B. sensor_msgs/Image

C. geometry_msgs/Twist

D. visualization_msgs/InteractiveMarker

Solution

1. Step 1: Identify message type for interactive markers

   The message type visualization_msgs/InteractiveMarker is designed to define interactive markers in ROS.

2. Step 2: Eliminate unrelated message types

   sensor_msgs/Image is for images, geometry_msgs/Twist for velocity commands, std_msgs/String for text messages, so they don't create interactive markers.

3. Final Answer:

   visualization_msgs/InteractiveMarker -> Option D

4. Quick Check:

   Interactive marker message = visualization_msgs/InteractiveMarker [OK]

Hint: Interactive markers use visualization_msgs/InteractiveMarker type [OK]

Common Mistakes:

• Choosing geometry_msgs/Twist which is for velocity commands
• Confusing image or string messages with markers
• Not knowing ROS message types

3. Given this snippet handling interactive marker feedback in ROS:

void processFeedback(const visualization_msgs::InteractiveMarkerFeedbackConstPtr &feedback) {
  if (feedback->event_type == visualization_msgs::InteractiveMarkerFeedback::POSE_UPDATE) {
    geometry_msgs::Pose new_pose = feedback->pose;
    // Update robot command with new_pose
  }
}

What happens when the user moves the marker?

medium

A. The robot command updates with the marker's new pose

B. The marker resets to its original position

C. The feedback event_type is ignored

D. The robot stops moving immediately

Solution

1. Step 1: Analyze feedback event type

   The code checks if the event_type is POSE_UPDATE, which means the marker was moved or rotated.

2. Step 2: Understand the effect of POSE_UPDATE

   When POSE_UPDATE occurs, the new pose is extracted and used to update the robot command, so the robot moves accordingly.

3. Final Answer:

   The robot command updates with the marker's new pose -> Option A

4. Quick Check:

   POSE_UPDATE triggers robot command update [OK]

Hint: POSE_UPDATE means marker moved, update robot pose [OK]

Common Mistakes:

• Assuming marker resets automatically
• Ignoring the event_type check
• Thinking robot stops without command

4. You wrote this callback for interactive marker feedback but the robot does not move:

void feedbackCallback(const visualization_msgs::InteractiveMarkerFeedbackConstPtr &feedback) {
  if (feedback->event_type = visualization_msgs::InteractiveMarkerFeedback::POSE_UPDATE) {
    // update robot command
  }
}

What is the error?

medium

A. Missing return statement in the callback

B. Using assignment '=' instead of comparison '==' in the if condition

C. Incorrect message type for feedback parameter

D. Not publishing the updated robot command

Solution

1. Step 1: Check the if condition syntax

   The condition uses '=' which assigns a value instead of '==' which compares values.

2. Step 2: Understand effect of assignment in if

   Assignment always returns true, so the condition is always true but does not properly check event_type, causing logic errors.

3. Final Answer:

   Using assignment '=' instead of comparison '==' in the if condition -> Option B

4. Quick Check:

   Use '==' to compare event_type, not '=' [OK]

Hint: Use '==' to compare, '=' assigns value [OK]

Common Mistakes:

• Confusing '=' and '==' in conditions
• Assuming missing return causes no movement
• Ignoring need to publish commands

5. You want to create an interactive marker that allows the user to rotate a robot arm joint only around the Z axis. Which control type and orientation should you use in your marker setup?

hard

A. Use a BUTTON control with no orientation

B. Use a MOVE_PLANE control with orientation aligned to the X axis

C. Use a ROTATE_AXIS control with orientation set to align Z axis with the joint axis

D. Use a MOVE_AXIS control aligned to the Y axis

Solution

1. Step 1: Identify control type for rotation

   ROTATE_AXIS control allows rotation around a single axis, suitable for rotating a joint.

2. Step 2: Set orientation to align with joint axis

   To rotate around Z axis, set the control's orientation so its Z axis matches the joint's rotation axis.

3. Final Answer:

   Use a ROTATE_AXIS control with orientation set to align Z axis with the joint axis -> Option C

4. Quick Check:

   ROTATE_AXIS + Z orientation = rotate joint around Z [OK]

Hint: ROTATE_AXIS control with Z orientation rotates joint around Z [OK]

Common Mistakes:

• Using MOVE_PLANE which moves in a plane, not rotate
• Choosing BUTTON which is for clicks, not rotation
• Aligning control to wrong axis