# ACT (Action Chunking with Transformers)

ACT is a **lightweight and efficient policy for imitation learning**, especially well-suited for fine-grained manipulation tasks. It's the **first model we recommend when you're starting out** with LeRobot due to its fast training time, low computational requirements, and strong performance.

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_Watch this tutorial from the LeRobot team to learn how ACT works: [LeRobot ACT Tutorial](https://www.youtube.com/watch?v=ft73x0LfGpM)_

## Model Overview

Action Chunking with Transformers (ACT) was introduced in the paper [Learning Fine-Grained Bimanual Manipulation with Low-Cost Hardware](https://arxiv.org/abs/2304.13705) by Zhao et al. The policy was designed to enable precise, contact-rich manipulation tasks using affordable hardware and minimal demonstration data.

### Why ACT is Great for Beginners

ACT stands out as an excellent starting point for several reasons:

- **Fast Training**: Trains in a few hours on a single GPU
- **Lightweight**: Only ~80M parameters, making it efficient and easy to work with
- **Data Efficient**: Often achieves high success rates with just 50 demonstrations

### Architecture

ACT uses a transformer-based architecture with three main components:

1. **Vision Backbone**: ResNet-18 processes images from multiple camera viewpoints
2. **Transformer Encoder**: Synthesizes information from camera features, joint positions, and a learned latent variable
3. **Transformer Decoder**: Generates coherent action sequences using cross-attention

The policy takes as input:

- Multiple RGB images (e.g., from wrist cameras, front/top cameras)
- Current robot joint positions
- A latent style variable `z` (learned during training, set to zero during inference)

And outputs a chunk of `k` future action sequences.

## Installation Requirements

1. Install LeRobot by following our [Installation Guide](./installation).
2. ACT is included in the base LeRobot installation, so no additional dependencies are needed!

## Training ACT

ACT works seamlessly with the standard LeRobot training pipeline. Here's a complete example for training ACT on your dataset:

```bash
lerobot-train \
  --dataset.repo_id=${HF_USER}/your_dataset \
  --policy.type=act \
  --output_dir=outputs/train/act_your_dataset \
  --job_name=act_your_dataset \
  --policy.device=cuda \
  --wandb.enable=true \
  --policy.repo_id=${HF_USER}/act_policy
```

### Training Tips

1. **Start with defaults**: ACT's default hyperparameters work well for most tasks
2. **Training duration**: Expect a few hours for 100k training steps on a single GPU
3. **Batch size**: Start with batch size 8 and adjust based on your GPU memory

### Train using Google Colab

If your local computer doesn't have a powerful GPU, you can utilize Google Colab to train your model by following the [ACT training notebook](./notebooks#training-act).

## Evaluating ACT

Once training is complete, you can evaluate your ACT policy using the `lerobot-record` command with your trained policy. This will run inference and record evaluation episodes:

```bash
lerobot-record \
  --robot.type=so100_follower \
  --robot.port=/dev/ttyACM0 \
  --robot.id=my_robot \
  --robot.cameras="{ front: {type: opencv, index_or_path: 0, width: 640, height: 480, fps: 30}}" \
  --display_data=true \
  --dataset.repo_id=${HF_USER}/eval_act_your_dataset \
  --dataset.num_episodes=10 \
  --dataset.single_task="Your task description" \
  --dataset.streaming_encoding=true \
  --dataset.encoder_threads=2 \
  # --dataset.camera_encoder.vcodec=auto \
  --policy.path=${HF_USER}/act_policy
```
