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FR3 and Robotiq control

openpi-control owns one Franka Emika FR3 and one Robotiq 2F gripper through the same ArmSession and FollowerArm API used by the other follower arms. One pi_control_node process owns both hardware connections.

Requirements

  • libfranka is pinned to 0.21.3 and built from source by scripts/build_deps.sh.
  • The FR3 must run Robot System version 5.9.0 or newer (robot server protocol 10).
  • The host must be able to reach the FR3 controller over Ethernet.
  • The gripper must use either true Modbus RTU over a serial device or true Modbus TCP. RTU-over-TCP gateways are not treated as serial devices.

A real-time kernel is not required. The driver always constructs libfranka with franka::RealtimeConfig::kIgnore; the torque callback still runs at the 1 kHz rate owned by libfranka.

Python configuration

For serial RTU:

from openpi_control import ArmConfig, FR3Connection, RobotiqConnection

config = ArmConfig(
    "follower",
    "FR3",
    FR3Connection("192.168.1.10"),
    effector_model="Robotiq",
    effector_connection=RobotiqConnection.rtu(
        "/dev/serial/by-id/usb-robotiq",
        baud_rate=115200,
        slave_id=9,
    ),
)

For Modbus TCP, only the gripper connection changes:

effector_connection=RobotiqConnection.tcp("192.168.1.11", port=502)

The public gripper convention is 0.0 = fully closed and 1.0 = fully open. Raw Robotiq register calibration defaults to 3 (open) and 230 (closed).

Connecting is passive: the arm holds its measured pose and the gripper is not activated. move_to_ready() performs internal FR3 error recovery, moves to the configured seven-joint reset pose, activates the gripper, and waits for it to open fully. There are no FR3-specific activation, recovery, velocity-command, read-only, or synthetic backend APIs.

from openpi_control import ArmSession, PositionCommand

with ArmSession() as session:
    follower = session.add_follower(config)
    session.connect()
    follower.move_to_ready()
    follower.command(
        PositionCommand(
            [0.0, -0.6283185307, 0.0, -2.5132741229, 0.0, 1.8849555922, 0.0],
            1.0,
        )
    )

Native controller

Policy targets may arrive at a lower frequency while libfranka continues its 1 kHz torque callback. Position targets remain active until replaced or held. The controller retains the ported, tuned hybrid joint/Cartesian impedance gains:

  • Cartesian stiffness: 400, 400, 400, 15, 15, 15
  • Cartesian damping: 37, 37, 37, 2, 2, 2
  • Joint stiffness: 40, 30, 50, 25, 35, 25, 10
  • Joint damping: 4, 6, 5, 5, 3, 2, 1

Joint, velocity, Cartesian, torque, collision, owner-liveness, and command shape checks remain active. libfranka supplies the robot dynamics and kinematics, so the FR3 configuration does not require a packaged URDF.

The dependency builder produces static libfranka.a and libmodbus.a archives. Both are linked directly into the packaged pi_control_node, just like the existing pinned native dependencies.

Hardware acceptance

Perform the first actuating checks with the workspace clear and an operator at the E-stop:

  1. Connect and verify seven joint states plus one gripper state without motion.
  2. Verify the RTU or TCP gripper endpoint and the open/closed convention.
  3. Call move_to_ready() and verify the blocking reset motion and activation.
  4. Send low-rate position targets and verify the 1 kHz callback remains healthy.
  5. Terminate the Python owner and verify that the arm holds and gripper motion stops.