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Enhancing Robustness in Deep Reinforcement Learning: A Lyapunov Exponent Approach
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Shows deep RL policies (SAC, TD3, Dreamer V3) are chaotic under small state perturbations in DeepMind Control tasks, and proposes adding a variance-based Maximal Lyapunov Exponent regulariser to Dreamer V3's policy loss to improve robustness to observation noise.
Problem
Deep RL policies for continuous control are sensitive to small state perturbations and observation noise; their closed-loop dynamics can be chaotic, making rewards and trajectories unpredictable and fragile in real-world use.
Approach
Estimate the Maximal Lyapunov Exponent of the learned policy via variance across stochastic RSSM rollouts in Dreamer V3, and add this variance term as a regulariser to the policy loss so the agent favours stable state trajectories while maximising return.
Key results
- MLE regularisation reduced Dreamer V3's MLE in most environments, e.g. Walker Stand from 0.1688 to 0.0654 and Pointmass from 0.0326 to -0.0275, while reward stayed similar in most tasks.
- Under Gaussian observation noise, MLE DR3 significantly improved performance in four of six noisy control systems, while Walker Stand and Cheetah Run were similar to baseline.
- All deep RL policies (SAC, TD3, Dreamer V3) showed positive MLE in high-dimensional Walker and Cheetah tasks, indicating chaotic state dynamics.
Measured effects reported in the paper
| What was changed | Where | Measured | Effect |
|---|---|---|---|
| add MLE regularisation | Dreamer V3 · Pointmass | reward: 869.5 -> 880.5 | Helped |
| add MLE regularisation | Dreamer V3 · Cartpole Balance | reward: 978.6 -> 970.5 | Hurt |
| add MLE regularisation | Dreamer V3 · Cartpole Swingup | reward: 781.4 -> 866.4 | Helped |
| add MLE regularisation | Dreamer V3 · Walker Stand | reward: 973.0 -> 961.6 | Hurt |
| add MLE regularisation | Dreamer V3 · Walker Walk | reward: 948.6 -> 950.7 | Helped |
| add MLE regularisation | Dreamer V3 · Walker Run | reward: 646.3 -> 698.4 | Helped |
| add MLE regularisation | Dreamer V3 · Cheetah Run | reward: 737.7 -> 675.2 | Hurt |
| add MLE regularisation | Dreamer V3 · Pointmass | MLE: 0.0326 -> -0.0275 | Helped |
| add MLE regularisation | Dreamer V3 · Cartpole Balance | MLE: 0.0249 -> ~0.0231 | Helped |
| add MLE regularisation | Dreamer V3 · Cartpole Swingup | MLE: 0.0149 -> ~0.0235 | Hurt |
| add MLE regularisation | Dreamer V3 · Walker Stand | MLE: 0.1688 -> ~0.0654 | Helped |
| add MLE regularisation | Dreamer V3 · Walker Walk | MLE: 0.1614 -> ~0.1405 | Helped |
Surprising findings
- Cartpole Balance and Cartpole Swingup policies learned by SAC and TD3 were stable, but Dreamer V3 produced low-level chaos even in these simple environments.
- Dreamer V3 with MLE regularisation still had positive MLE in most environments, so the trajectories remained chaotic though less divergent.
Limitations
- Regularisation assumes RSSM accurately estimates local state divergence; may be less effective where the RSSM struggles to model dynamics.
- MLE remains positive in most environments after regularisation, so trajectories are still chaotic.
- Only L=3 samples used for variance estimation due to compute constraints.
Experimental setup
Datasets: DeepMind Control Suite Pointmass Cartpole Balance Cartpole Swingup Walker Stand Walker Walk Walker Run Cheetah Run
Benchmarks: DeepMind Control Suite
Metrics: episode reward maximal Lyapunov exponent sum of Lyapunov exponents reward MLE interquartile mean reward under Gaussian observation noise
Baselines: SAC TD3 Dreamer V3
Builds on: Dreamer V3 Soft Actor-Critic Benettin et al. Lyapunov spectrum estimation Stable Baselines3
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