a2_unitree_bridge

ROS 2 bridge between the Unitree DDS layer and ROS 2 topics for the A2 robot. Two build targets share the same source — one for the real robot (HG firmware) and one for MuJoCo simulation.

Build

colcon build --packages-select a2_unitree_bridge

Run

Simulation (MuJoCo + Unitree DDS on loopback):

ros2 launch a2_unitree_bridge sim.launch.py

Real robot (ethernet, HG firmware):

ros2 launch a2_unitree_bridge robot.launch.py

Topics

Sim (a2_bridge_sim)

Topic	Type	Rate	Notes
/clock	rosgraph_msgs/Clock	DDS rate	Sim time derived from MuJoCo tick
/joint_states	sensor_msgs/JointState	200 Hz	Throttled from LowState DDS
/imu/data	sensor_msgs/Imu	200 Hz	Throttled from LowState DDS
/odom	nav_msgs/Odometry	50 Hz	Ground-truth pose from SportModeState
/state_estimation	nav_msgs/Odometry	50 Hz	Same as /odom (for downstream compatibility)
/a2/sport_mode	std_msgs/UInt8	50 Hz	Current sport mode enum value
/camera/image_raw	sensor_msgs/Image	DDS rate	Front camera RGB decoded from PointCloud2 DDS
/camera/camera_info	sensor_msgs/CameraInfo	DDS rate	Intrinsics from a2_description/config/camera_info_sim.yaml, published with each image
/front_lidar/points	sensor_msgs/PointCloud2	DDS rate	Raw front lidar scan
/rear_lidar/points	sensor_msgs/PointCloud2	DDS rate	Raw rear lidar scan
/lowcmd (subscribed)	unitree_go/LowCmd	—	Receives low-level joint commands and forwards to DDS

TF: map → base_link broadcast at 50 Hz from SportModeState ground-truth position.

Robot (a2_bridge_robot)

Topic	Type	Rate	Notes
/joint_states	sensor_msgs/JointState	200 Hz	Throttled from LowState DDS (HG firmware)
/imu/data	sensor_msgs/Imu	200 Hz	Throttled from LowState DDS
/a2/sport_mode	std_msgs/UInt8	50 Hz	Current sport mode enum value
/a2/mode (subscribed)	a2_interfaces/msg/OperatingMode	—	Requests FSM mode transitions
/cmd_vel (subscribed)	geometry_msgs/TwistStamped	—	Velocity commands; must be stamped, dropped if >500 ms old

Odometry on the real robot comes from DLIO, not this bridge.

Operating the robot (a2_bridge_robot)

The robot is driven through a mode state machine (FSM) defined in a2_utils. Modes must be stepped through in order — you cannot jump directly from STAND_DOWN to VELOCITY_MOVE.

Mode transitions

Publish to /a2/mode (a2_interfaces/msg/OperatingMode, field mode: uint8) to request a transition:

Value	Mode	Description
0	ESTOP	Damp all joints immediately. Reachable from any mode.
1	STAND_DOWN	Sit down, joints locked. Starting state on launch.
2	STAND_UP	Stand up from STAND_DOWN.
3	BALANCE_STAND	Active balance stand from STAND_UP.
4	VELOCITY_MOVE	Accept velocity commands. Reachable from BALANCE_STAND.
5	FREE	Stop motion without disabling joints. Reachable from any mode.

Valid transition sequence to start moving:

STAND_DOWN → STAND_UP → BALANCE_STAND → VELOCITY_MOVE

From FREE, the robot can return to whatever mode it was in before FREE was requested, or transition to STAND_DOWN.

Example (CLI):

ros2 topic pub --once /a2/mode a2_interfaces/msg/OperatingMode "{mode: 1}"  # STAND_DOWN
ros2 topic pub --once /a2/mode a2_interfaces/msg/OperatingMode "{mode: 2}"  # STAND_UP
ros2 topic pub --once /a2/mode a2_interfaces/msg/OperatingMode "{mode: 3}"  # BALANCE_STAND
ros2 topic pub --once /a2/mode a2_interfaces/msg/OperatingMode "{mode: 4}"  # VELOCITY_MOVE

Velocity commands

Once in VELOCITY_MOVE, publish geometry_msgs/TwistStamped to /cmd_vel. Commands older than 500 ms (by header stamp) are dropped, and velocity is zeroed after each control tick — the publisher must send continuously at the desired rate.

Field	Axis	Limit
twist.linear.x	Forward/back	±0.15 m/s
twist.linear.y	Left/right	±0.10 m/s
twist.angular.z	Yaw rate	±0.10 rad/s

The control loop runs at 50 Hz.