LunarLander-v2

PPO Agent playing LunarLander-v2

This is a trained Proximal Policy Optimization (PPO) agent playing LunarLander-v2.

This project was completed as part of the Hugging Face Deep Reinforcement Learning Course, Unit 8 - Part 1.

The PPO agent was implemented from scratch using PyTorch and Gymnasium, following the PPO implementation and concepts covered in the course.

Environment

The agent was trained on LunarLander-v2.

The goal is to learn a policy that controls a lunar lander and successfully lands it on the landing pad while maximizing the cumulative reward.

The environment provides an 8-dimensional observation describing:

• Horizontal position
• Vertical position
• Horizontal velocity
• Vertical velocity
• Angle
• Angular velocity
• Left leg contact
• Right leg contact

The action space contains four discrete actions:

Algorithm

The agent uses Proximal Policy Optimization (PPO).

PPO is an on-policy policy-gradient reinforcement learning algorithm that improves the policy while limiting excessively large policy updates.

The implementation includes:

• Actor-Critic architecture
• Generalized Advantage Estimation (GAE)
• PPO clipped surrogate objective
• Clipped value loss
• Advantage normalization
• Entropy regularization
• Gradient clipping
• Learning-rate annealing

PPO Clipped Objective

The probability ratio between the current and old policies is:

r_t(theta) = pi_theta(a_t | s_t) / pi_theta_old(a_t | s_t)

The PPO clipped objective is:

L_CLIP = E[min(r_t A_t, clip(r_t, 1-epsilon, 1+epsilon) A_t)]

The clipping coefficient used is:

epsilon = 0.2

Therefore, the clipping range is:

[0.8, 1.2]

Clipping prevents the policy from making excessively large updates.

When the ratio is within the clipping range, the policy can be updated normally.

When the ratio moves outside the range in a direction that would make the policy update excessively large, the clipped objective limits the update.

Generalized Advantage Estimation

The implementation uses Generalized Advantage Estimation (GAE).

The temporal-difference error is:

delta_t = r_t + gamma * V(s_t+1) - V(s_t)

The advantage is estimated recursively using:

A_t = delta_t + gamma * lambda * A_t+1

The implementation uses:

• Gamma = 0.99
• GAE Lambda = 0.95

GAE provides a balance between bias and variance when estimating advantages.

Model Architecture

The PPO agent uses an Actor-Critic architecture.

Actor

The Actor receives the environment observation and produces action logits.

Architecture:

Input -> Linear(64) -> Tanh -> Linear(64) -> Tanh -> Linear(4)

The output is used to create a categorical probability distribution over the four possible actions.

Critic

The Critic estimates the value of the current state.

Architecture:

Input -> Linear(64) -> Tanh -> Linear(64) -> Tanh -> Linear(1)

Training Configuration

Evaluation

The trained agent was evaluated for 10 episodes.

The evaluation results are stored in evaluation.txt.

The file contains:

• Number of evaluation episodes
• Mean reward
• Standard deviation
• Individual episode rewards

The evaluation result shown at the top of this model card should be replaced with the actual mean reward from evaluation.txt.

Usage

The trained model is stored as model.pt.

The model contains the PyTorch state dictionary of the trained Actor-Critic agent.

The same Agent architecture must be recreated before loading the weights.

Example:

import torch

agent.load_state_dict(
    torch.load(
        "model.pt",
        map_location="cpu"
    )
)

agent.eval()

The environment can be created using:

import gymnasium as gym

env = gym.make("LunarLander-v2")

observation, info = env.reset()

Repository Contents

• model.pt - trained PPO Actor-Critic model
• hyperparameters.txt - PPO training hyperparameters
• evaluation.txt - evaluation results
• README.md - model card

Hugging Face Deep Reinforcement Learning Course

This project was completed as part of the:

Hugging Face Deep Reinforcement Learning Course

Unit 8 - Part 1: Proximal Policy Optimization (PPO)

The project demonstrates the implementation of PPO from scratch and its application to the LunarLander environment.

The main learning objectives include:

• Understanding PPO
• Implementing an Actor-Critic architecture
• Collecting experience from multiple environments
• Implementing Generalized Advantage Estimation
• Implementing the PPO clipped objective
• Training and evaluating the agent
• Sharing the trained model on the Hugging Face Hub

References

• Hugging Face Deep Reinforcement Learning Course
• Proximal Policy Optimization Algorithms - Schulman et al.
• Gymnasium
• PyTorch

Limitations

This model was developed as an educational project for the Hugging Face Deep Reinforcement Learning Course.

Performance may vary depending on:

• Random seed
• Training duration
• Hyperparameters
• Environment version
• Hardware
• Stochasticity of the environment