microduck-swing

This MicroDuck policy starts motionless at the bottom of a two-cord swing and pumps with its articulated head and legs. The best strict 36-second rollout reaches a 173.20° full span; 71 of 100 randomized seeds pass every full-horizon physical-validity gate.

The complete task, PPO configuration, selected PyTorch checkpoint and selection endpoints, deterministic evaluator, seat generator, printable meshes, and collision hulls are in Vottivott/microduck-playground at commit c5fcc50.

Retained seat

The printable seat, strap, buckle, clearance reports, and parametric source are in hardware/swing-seat.

Try it in simulation

git clone https://github.com/Vottivott/microduck-playground.git
cd microduck-playground
git checkout c5fcc50219fef01ac9931d0079c583ccbb29b689
uv sync

# Reproduce the best strict 36-second audit from the source checkpoint.
uv run python scripts/evaluate_swing_checkpoint.py \
  experiments/swing/checkpoints/alpha050.pt \
  --output /tmp/swing-seed27.json \
  --device cpu --duration 36 --seed 27

# Download the deployment graph separately.
hf download HannesVonEssen/microduck-swing policy.onnx config.json \
  --local-dir policies/swing

Continue training

checkpoint.pt contains the exact alpha-0.50 actor used in this video. Its critic is copied from the interpolation source endpoint; optimizer moments are empty, the learning rate is 1e-7, and exploration standard deviation is 0.02. Place it at logs/rsl_rl/microduck_swing/release-alpha050/model_3500.pt, then resume with:

uv run train Mjlab-SwingPump-MicroDuck \
  --agent.resume True --agent.load-run release-alpha050 \
  --agent.load-checkpoint model_3500.pt --agent.max-iterations 25

The released lineage ends here, at the policy shown in the 173.20° video. PyTorch checkpoints use pickle internally; load them only from a repository and revision you trust.

Runtime contract

• input: obs, float32 [1, 61]
• output: actions, float32 [1, 14]
• control rate: 50 Hz
• action scale: 0.7 rad, joint-position targets around MicroDuck HOME
• action clipping: [-1, 1], baked into policy.onnx to match training
• observation normalizer: baked into policy.onnx
• entry state: retained in the seat, bottom of the arc, motionless
• robot: MicroDuck hardware revision 1, 14 Dynamixel XL330 servos
• mechanism: two independent 380 mm elastic, tension-only cords
• policy horizon: trained episodically; evaluated for 36 seconds

The observation order is base_ang_vel(3), projected_gravity(3), joint_pos(14), joint_vel(14), previous_actions(14), swing_plane_cue(3), zeros(4), zeros(6).

Important: the three cue slots are not velocity commands

This model preserves the standard 61-D MicroDuck tensor shape, but repurposes the usual three twist-command slots as a deployable swing-plane feedback cue:

[0, body_y_axis_world.x, body_y_axis_world.z]

body_y_axis_world must come from the robot's IMU orientation expressed relative to the known still-start frame. The remaining 10 command slots are zeros. A standard walking runtime that supplies a requested velocity in these slots is therefore incompatible without this small observation adapter. No cord, pivot, camera, or motion-capture measurement is supplied to the actor.

Simulation evaluation

The selected model is the exact all-row alpha-0.50 final-layer interpolation documented in the source repository. Selection uses mechanism validity rather than angle alone.

Strict screening rejects a rollout for excessive lateral displacement, attachment misalignment, deep cord slack, overextension, NaNs, reset, or accumulated geometry debt. The preview is the strict seed-27 rollout; it is silent and the overlay reports the running maximum full-span angle.

Sim-to-real boundary

The task retains deployment-oriented MicroDuck modeling, including BAM XL330 voltage/back-EMF/current behavior, battery variation and load-dependent sag, control delay, actuator friction variation, encoder bias, IMU/encoder noise, and two independent elastic tension-only cords. The actor uses only IMU/encoder/action-history signals available on the robot.

This is a simulation result, not a hardware-validated policy. It requires the retained swing seat, 380 mm cords, and a suitable rigid frame. Cord knots, frame flex, textile strap contact, seat padding, real collision geometry, servo temperature, assembly tolerances, and the IMU-frame calibration are not fully captured. Use a separate safety tether, current limits, an emergency stop, a clear exclusion zone, and conservative incremental testing.