Joint GUI

The Joint GUI is a Tkinter application that provides interactive slider-based control of robot joints during simulation. It launches automatically as part of the ROS 2 launch file.

How it works

Startup sequence

1. Parse the URDF for all revolute joints
   → Extract joint names, lower/upper limits
   
2. Subscribe to /joint_states
   → Wait for first message to discover active joints
   → Record origin angles for each joint
   
3. Build the GUI
   → Create sliders with limits from the URDF
   → Auto-discover available JointTrajectory topics
   
4. Run two threads
   → Main thread: Tkinter event loop (UI updates)
   → Background thread: ROS 2 spin (message callbacks)

Joint discovery

The GUI doesn't hardcode joint names. Instead:

1. It parses the robot's URDF file (passed as an argument) for all joints of type revolute
2. It subscribes to /joint_states and waits for the first message to arrive
3. The joints reported in that message become the active joints shown in the GUI

This means the GUI works with any robot -- it adapts to whatever joints exist.

Origin angles

When joints are first discovered, their current positions (from /joint_states) are saved as "origin angles." Slider values are offsets from these origins. When you hit Send, the published position is slider_value + origin_angle.

This matters because joints may not start at position 0 -- the origin angle ensures the slider's zero position matches the robot's natural resting state.

Controls

Control	Description
Topic dropdown	Select which JointTrajectory topic to publish on. Auto-populated from available topics.
Refresh	Re-scan for available trajectory topics
Joint sliders	One per joint. Range is set by the URDF's <limit lower="..." upper="..."> values.
Duration	Time in seconds for the trajectory to complete (default: 2.0s)
Send	Publish current slider positions as a trajectory command
Sync	Read current positions from /joint_states and update sliders to match
Reset	Zero all sliders (back to origin position)

ROS 2 interface

Subscribes to:

• /joint_states (sensor_msgs/msg/JointState) -- reads current joint positions

Publishes to:

• Configurable trajectory topic (default: /joint_trajectory_controller/joint_trajectory)
• Message type: trajectory_msgs/msg/JointTrajectory

Running standalone

The GUI is registered as a console script entry point. You can run it outside of the launch file:

Inside devcontainer

ros2 run sim_common joint_gui <path_to_urdf>

For example:

Inside devcontainer

ros2 run sim_common joint_gui \
    $(ros2 pkg prefix arm_description)/share/arm_description/urdf/arm.urdf

The simulation must already be running with controllers active for the GUI to discover joints.

Source

The implementation is in gazebo/sim_common/sim_common/joint_gui.py. Key classes:

• JointPublisher (ROS 2 Node) -- handles URDF parsing, joint state subscription, topic discovery, and trajectory publishing
• JointGui (Tkinter) -- builds the UI, manages sliders, and dispatches commands to JointPublisher