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Chance-Constrained POMDP Planning using Learned Probabilistic Failure Surrogates and Adaptive Safety Constraints


Core Concepts
The core message of this work is to introduce ConstrainedZero, an extension of the BetaZero POMDP planning algorithm that solves chance-constrained POMDPs (CC-POMDPs) by learning neural network approximations of the optimal value, policy, and failure probability, and using an adaptive Monte Carlo tree search (∆-MCTS) to plan under safety constraints.
Abstract
This paper introduces the ConstrainedZero algorithm for solving chance-constrained partially observable Markov decision processes (CC-POMDPs). The key elements are: ConstrainedZero extends the BetaZero POMDP planning algorithm by adding a neural network head that estimates the failure probability given a belief. This failure probability is used to guide safe action selection during online Monte Carlo tree search (MCTS). To avoid overemphasizing search based on the failure estimates, the authors introduce ∆-MCTS, which uses adaptive conformal inference to update the failure probability threshold during planning. This allows ConstrainedZero to balance safety and utility. The approach is evaluated on three safety-critical POMDP benchmarks: a long-horizon localization task (LightDark), an aircraft collision avoidance system, and the sustainability problem of safe CO2 storage. The results show that by separating safety constraints from the objective, ConstrainedZero can achieve a target level of safety without optimizing the balance between rewards and costs. The key benefit of ConstrainedZero is that it can plan safely in uncertain environments by learning neural network approximations of the optimal value, policy, and failure probability, and using an adaptive MCTS algorithm to select actions that satisfy safety constraints.
Stats
The paper presents the following key metrics and figures: For the LightDark localization task, ConstrainedZero achieves a failure probability of 0.01 ± 0.01 while maximizing returns, outperforming the Pareto frontier of the baseline BetaZero algorithm. For the aircraft collision avoidance system, ConstrainedZero satisfies the safety constraint earlier during policy iteration compared to BetaZero, while also maximizing returns. For the safe CO2 storage problem, ConstrainedZero achieves a failure probability of 0.05 ± 0.02 and a return of 2.62 ± 0.12.
Quotes
"ConstrainedZero exceeds the BetaZero Pareto curve and achieves the target level of safety with a failure probability of 0.01 ± 0.01 computed over 100 episodes." "With adaptation, ConstrainedZero adjusts the constraint in response to feedback from the environment, resulting in the algorithm becoming more capable at optimizing its performance within the bounds of the adaptive constraint."

Deeper Inquiries

What other safety-critical domains could benefit from the ConstrainedZero algorithm, and how would the approach need to be adapted to handle the specific challenges of those domains

Safety-critical domains such as autonomous driving, medical decision-making, and industrial automation could benefit from the ConstrainedZero algorithm. In autonomous driving, ConstrainedZero could help vehicles navigate complex environments while ensuring safety constraints are met, such as maintaining a safe distance from other vehicles or pedestrians. The algorithm would need to be adapted to handle the real-time nature of autonomous driving, incorporating sensor data and reacting quickly to changing conditions. In medical decision-making, ConstrainedZero could assist in treatment planning, ensuring patient safety while optimizing treatment outcomes. The algorithm would need to consider uncertainties in patient conditions and treatment responses, adapting the safety constraints to account for potential risks in medical interventions. For industrial automation, ConstrainedZero could be used to optimize production processes while ensuring worker safety and equipment reliability. The algorithm would need to handle dynamic environments, equipment failures, and human-machine interactions, adjusting safety constraints accordingly.

How could the ∆-MCTS algorithm be extended to handle multiple failure modes or more complex safety constraints beyond a single probability threshold

To extend the ∆-MCTS algorithm to handle multiple failure modes or complex safety constraints, the approach could be modified to incorporate a more sophisticated failure probability model. Instead of a single threshold for failure probability, the algorithm could consider different thresholds for distinct failure modes or types of safety violations. This would involve enhancing the failure probability estimation network to differentiate between various failure scenarios and adjusting the safety constraints based on the specific risk associated with each mode. Additionally, the adaptation mechanism in ∆-MCTS could be enhanced to dynamically update multiple thresholds based on the observed failure probabilities for different modes. By incorporating a more nuanced understanding of failure modes and their associated risks, the algorithm could provide more tailored and effective safety guarantees in complex environments.

Can the ConstrainedZero framework be applied to fully observable Markov decision processes (MDPs) to enable safe planning in deterministic or known environments, and how would the approach differ from the POMDP setting

The ConstrainedZero framework can be applied to fully observable Markov decision processes (MDPs) to enable safe planning in deterministic or known environments by focusing on optimizing policies while satisfying safety constraints. In fully observable MDPs, the algorithm would not need to maintain a belief state, as the agent has complete information about the environment state. In this setting, ConstrainedZero would prioritize actions based on the known state of the environment and the safety constraints defined for specific states or actions. The adaptation mechanism could be adjusted to update safety thresholds based on deterministic outcomes and known risks associated with different actions. Overall, the approach in fully observable MDPs would involve optimizing policies directly based on the known state information, while ensuring that safety constraints are met in every decision made by the agent. This would require a different formulation of the safety constraints and adaptation mechanism compared to the partially observable setting of POMDPs.
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