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  4. MAD: A Magnitude And Direction Policy Parametrization for Stability Constrained Reinforcement Learning
 
conference paper

MAD: A Magnitude And Direction Policy Parametrization for Stability Constrained Reinforcement Learning

Furieri, Luca  
•
Shenoy, Sucheth  
•
Saccani, Danilo  
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December 9, 2025
2025 IEEE 64th Conference on Decision and Control (CDC)
2025 IEEE 64th Conference on Decision and Control (CDC)

We introduce magnitude and direction (MAD) policies, a policy parameterization for reinforcement learning (RL) that preserves ℓp closed-loop stability for nonlinear dynamical systems. Despite their completeness in describing all stabilizing controllers, methods based on nonlinear Youla and system-level synthesis are significantly impacted by the difficulty of parametrizing ℓp-stable operators. In contrast, MAD policies introduce explicit feedback on state-dependent features – a key element behind the success of reinforcement learning pipelines – without jeopardizing closed-loop stability. This is achieved by letting the magnitude of the control input be described by a disturbance-feedback ℓp-stable operator, while selecting its direction based on state-dependent features through a universal function approximator. We further characterize the robust stability properties of MAD policies under model mismatch. Unlike existing disturbance-feedback policy parametrizations, MAD policies introduce state-feedback components compatible with model-free RL pipelines, ensuring closed-loop stability with no model information beyond assuming open-loop stability. Numerical experiments show that MAD policies trained with deep deterministic policy gradient (DDPG) methods generalize to unseen scenarios – matching the performance of standard neural network policies while guaranteeing closed-loop stability by design.

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