Small, low-inertia leader arms for robot teleoperation

Open Arms Mini is a compact, Feetech-based, 3D-printed leader arm for robot teleoperation. It has 7 DOF with human-like kinematics — 3 shoulder joints, 1 elbow, 1 forearm rotation, and 2 wrist joints — plus a gripper. Designed to be paired with the Open Arms follower arm and integrates with LeRobot as the openarm_mini teleoperator.

• Low inertia — quick, deliberate motions for precision tasks like cloth folding
• Arm-length agnostic — wrist-grip design works for teleoperators of any height
• Affordable — ~150 EUR per arm, ~300 EUR for a pair

The wrist strap makes a big difference for wrist roll precision — without it the hand tends to slip. Highly recommended.

See BOM.md for full details. ~150 EUR per arm / ~300 EUR for a pair (excl. filament and screws).

STL files: STL/ — STEP files (for modification): STEP/

Quantities are for a complete pair (left + right arm). Universal parts (no L/R) = 2, gripper L/R parts = 1 each, WaveShare plate shared = 1. Total: 32 parts for a pair (16 per arm).

Per arm: 8× Feetech STS3215-C046 servo (7.4 V, 1:147), 1× Waveshare Serial Bus Servo Driver Board, M2×6 and M3×6 screws (~35 each), 8× 3-pin servo cables, 7.5 V power supply, 1× velcro wrist strap (~25 mm).

Configure motors before assembly (easier when not yet installed):

lerobot-setup-motors \
    --teleop.type=openarm_mini \
    --teleop.port=/dev/tty.usbmodem575E0031751

Joint swap note: Wrist flex (6) and wrist roll (7) are deliberately swapped in software relative to the follower arm — this feels more natural given how the human wrist moves. Handled automatically by the openarm_mini driver.

1. Shoulder Pan (J1) — Insert motor 1, secure with 4× M2×6. Attach J1_holder with 2× M2×6. Install motor horns, secure top horn with M3×6.
2. Shoulder Lift (J2) — Slide motor 2 in, fasten with 4× M2×6. Attach motor horns. Connect J2 to J1 with 4× M3×6 each side.
3. Shoulder Roll (J3) — Insert motor 3, fasten with 4× M2×6. Attach motor horns. Connect J3 to J2 with 4× M3×6 each side.
4. Elbow (J4) — Slide J4_holder over J3. Insert motor 4, fasten with 4× M2×6. Attach motor horns.
5. Forearm Rotation (J5) — Insert motor 5, secure with 2× M2×6. Install one motor horn. Secure J5 to motor 4 with 4× M3×6 each side.
6. Wrist Flex (J6) — Attach J6 to motor 5 horn with 4× M3×6. Insert motor 6, secure with 2× M2×6. Attach motor horns.
7. Wrist Roll (J7) — Attach J7 to motor 6 with J7_holder. Insert motor 7, fasten with 4× M2×6.
8. Gripper — Insert motor 8 into J8 holders. Attach J8 L/R claws with 4× M3×6 each side. Attach triggers.
9. Handle & strap — Attach J Handle. Mount J6 holder with strap at the wrist. Thread velcro strap and adjust for a snug fit.
10. Controller board — Mount Waveshare board on WaveShare_Mounting_Plate. Daisy-chain motors 1→8.

Natively integrated into LeRobot — source: lerobot/teleoperators/openarm_mini.

pip install lerobot[feetech]

# Find port
lerobot-find-port

# Calibrate
lerobot-calibrate \
    --teleop.type=openarm_mini \
    --teleop.port=/dev/tty.usbmodem575E0031751 \
    --teleop.id=my_mini_leader_arm

# Teleoperate
lerobot-teleoperate \
    --teleop.type=openarm_mini \
    --teleop.port=/dev/tty.usbmodem575E0031751 \
    --teleop.id=my_mini_leader_arm

Hardware designs released under Apache 2.0. Based on the SO-ARM100 design.