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The simulation stack runs the full UR robot driver with use_mock_hardware:=true, which replaces the real hardware interface with an internal joint-state simulator. Gazebo Harmonic renders the robot in a 3D table-world environment and bridges clock and sensor topics into ROS 2. This lets you develop, test, and visualize motion planning without connecting to the physical arm.

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How mock hardware mode works

When use_mock_hardware:=true, the ur_robot_driver loads a mock hardware plugin instead of communicating over the network. The plugin publishes simulated joint states and accepts joint trajectory commands exactly as the real driver would. The robot_ip is set to 0.0.0.0 because no network connection is made. headless_mode is set to false so that all standard topics and services are available for debugging. Contrast this with real-hardware mode (launch-lan), where use_mock_hardware:=false, robot_ip:=192.168.56.101, and headless_mode:=true to connect to the physical arm.

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Simulation launch steps

1

Build the workspace

The sim task depends on build, so Pixi will run colcon build --symlink-install automatically if the install/ directory is out of date. You can also trigger a build explicitly:
pixi run build
2

Launch Gazebo with the mock driver

Both sim and mock are aliases for the same launch-sim task:
pixi run sim
# or equivalently
pixi run mock
This starts:
  • Gazebo Harmonic with the table_world.sdf scene
  • The UR robot driver in mock hardware mode
  • The robot_state_publisher with the Gazebo URDF
  • The ros_gz_bridge clock bridge
  • Joint state broadcaster and trajectory controller
Wait for the Gazebo window to open and the arm to appear in the scene before proceeding.
3

Open RViz with MoveIt 2

In a second terminal, launch the MoveIt 2 planning stack and RViz:
pixi run moveit
This runs:
ros2 launch ur_moveit_config ur_moveit.launch.py \
  ur_type:=ur3e \
  use_sim_time:=true \
  launch_rviz:=true
RViz opens with the MoveIt 2 motion planning panel. Use the interactive marker on the end effector to set a goal pose, then click Plan & Execute to send a trajectory to the simulated driver.

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Viewing the URDF

To inspect the robot description outside of a full simulation, use: 
pixi run view-ur
This launches urdf_tutorial display.launch.py, which opens RViz with just the robot model and a joint state publisher GUI. Useful for verifying URDF changes, checking link frames, or understanding the kinematic chain before running any controllers.

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Differences from real-hardware launch

ParameterSimulation (launch-sim)Real hardware (launch-lan)
use_mock_hardwaretruefalse
robot_ip0.0.0.0192.168.56.101
headless_modefalsetrue
launch_dashboard_clientfalsefalse
use_sim_timetruefalse
You can run the pick-and-place demo node against the simulated driver for motion testing, but the VacuumControl node’s digital I/O calls will fail without the real UR hardware interface. Comment out or mock the SetIO calls when testing purely in simulation.

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