---
library_name: lerobot
license: apache-2.0
pipeline_tag: robotics
tags:
- act
- diffusion
- robotics
- imitation-learning
- behavior-cloning
- aloha
- pytorch_model_hub_mixin
- model_hub_mixin
datasets:
- JHeisler/aloha_solo_left_4_6_26
---

# Hybrid ACT+Diffusion — ALOHA Single-Arm (Left) — 40k steps

Custom **HybridACTDiffusion** policy: ACT visual encoder (ResNet18 + 4-layer Transformer, mean-pooled) feeds a Diffusion U-Net decoder (FiLM conditioning, DDPM training, DDIM 10-step inference). No VAE — diffusion handles multimodal action distributions directly.

This is the **40k-step retrain (workstream S004)** matching S003's step count for direct architectural comparison vs the shipped ACT-40k baseline. For the initial 13.4k baseline, see [JHeisler/aloha_solo_left_act_diffusion](https://huggingface.co/JHeisler/aloha_solo_left_act_diffusion).

## Architecture

```
Images (cam_high, cam_left_wrist) + State (dim=9)
     │
     ▼
ACT Encoder (ResNet18 → 4-layer Transformer) → mean-pool → (B, 512) global cond vector
     │
     ▼
Diffusion U-Net (DiffusionConditionalUnet1d, FiLM modulation, down_dims=(256,512))
     │  DDPM training (100 timesteps) / DDIM 10-step inference
     ▼
Action chunks (chunk_size=100, action_dim=9)
```

## Training Config

| Field | Value |
|---|---|
| Architecture | HybridACTDiffusion (ACT encoder + Diffusion U-Net) — see `lerobot/common/policies/hybrid_act_diffusion/` |
| Dataset | [JHeisler/aloha_solo_left_4_6_26](https://huggingface.co/datasets/JHeisler/aloha_solo_left_4_6_26) — 50 episodes, 29,785 samples, 30 fps |
| State / action dim | 9 / 9 |
| Cameras | `cam_high`, `cam_left_wrist` (3×480×640 each) |
| Steps | 40,000 |
| Batch size | 28 (adaptive DOE winner — beats bs=24 by 6.8% throughput at 91.3 smpl/s) |
| Learning rate | 3.5e-5 (linear-scaled from bs=24's 3e-5) |
| Total samples seen | ~1.12M (~37 epochs over the dataset) |
| AMP | enabled |
| torch.compile | enabled |
| Save freq | every 10,000 steps (10k / 20k / 30k / 40k checkpoints) |
| Diffusion scheduler | DDPM training (100 timesteps, squaredcos_cap_v2), DDIM at inference (10 steps) |
| Final loss (DDPM noise-pred MSE) | 0.003–0.007 |
| Final grad norm | ~0.10–0.18 |
| Wall clock | ~3h 53min on RTX A4500 |
| LeRobot pin | `96c7052777aca85d4e55dfba8f81586103ba8f61` (with custom hybrid_act_diffusion policy added) |

## Project Lineage

| Workstream | Model | Steps | Samples | HF |
|---|---|---|---|---|
| S001 | ACT | 13,400 | 640K | [act_left](https://huggingface.co/JHeisler/aloha_solo_left_4_6_26_act_left) |
| S002 | Hybrid ACT+Diffusion | 13,400 | 321K | [act_diffusion](https://huggingface.co/JHeisler/aloha_solo_left_act_diffusion) |
| S003 | ACT (shipped) | 40,000 | 1.92M | [act_left_40k](https://huggingface.co/JHeisler/aloha_solo_left_4_6_26_act_left_40k) |
| **S004** | **Hybrid ACT+Diffusion** | **40,000** | **1.12M** | **this repo** |

S003 vs S004 is the apples-to-apples architectural comparison: same dataset, same step count, ACT-VAE vs ACT-Diffusion decoder.

## Notes on loss comparability

DDPM noise-prediction MSE (this model) and ACT's L1+KL combo (S001/S003) are different loss surfaces — absolute loss values are NOT directly comparable across architectures. The right comparison is offline action L1 on held-out episodes or real-robot rollout success rate.

## Usage

```python
# Requires lerobot pinned to 96c7052 with hybrid_act_diffusion policy package added
from lerobot.common.policies.hybrid_act_diffusion.modeling_hybrid_act_diffusion import HybridACTDiffusionPolicy
policy = HybridACTDiffusionPolicy.from_pretrained("JHeisler/aloha_solo_left_act_diffusion_40k")
```

## Citation / Course

EN.525.681 school project — JHU Whiting School of Engineering. Team: Jake Heisler, Laura Kroening, Purushottam Shukla.

Code reference: [HuggingFace LeRobot](https://github.com/huggingface/lerobot) at commit `96c7052` with custom hybrid policy package.