---
library_name: onnx
pipeline_tag: reinforcement-learning
tags: [microduck, microduck-policy, mjlab, robotics, reinforcement-learning, swing]
license: apache-2.0
---
# 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`](https://github.com/Vottivott/microduck-playground)
at commit
[`5eec677`](https://github.com/Vottivott/microduck-playground/commit/5eec677cbf0e092523ee9b33bda57e81b9115817).
## Try it in simulation
```bash
git clone https://github.com/Vottivott/microduck-playground.git
cd microduck-playground
git checkout 5eec677cbf0e092523ee9b33bda57e81b9115817
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 \
--local-dir policies/swing
```
## 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
- 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:
```text
[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.
| metric | result |
|---|---:|
| randomized evaluation | 100 seeds × 36 s |
| strict full-horizon passes | 71/100 |
| geometry-debt-free passes | 73/100 |
| median peak-to-peak span | 163.03° |
| median final-six half-cycle span | 161.09° |
| best strict rollout | seed 27, 173.20° |
| seed-27 cord envelope | 370.38–392.02 mm |
| seed-27 maximum lateral displacement | 10.35 mm |
| seed-27 maximum attachment-alignment penalty | 0.02037 |
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.
