# Passive-Aggressive Learning and Control

## TL;DR

This paper introduces a passive-aggressive control method for scalar systems that adaptively learns system parameters while ensuring stability under adversarial disturbances, balancing exploration and exploitation.

## Contribution

It presents a novel controller that simultaneously stabilizes the system and learns unknown parameters, especially under adversarial conditions, with a natural optimization-based design.

## Key findings

- Controller guarantees global stability and improves bounds on state deviation.
- Effective in stabilizing unstable, adversarial system dynamics.
- Demonstrated efficiency through numerical simulations.

## Abstract

In this work, we investigate the problem of simultaneously learning and controlling a system subject to adversarial choices of disturbances and system parameters. We study the problem for a scalar system with $l_\infty$-norm bounded disturbances and system parameters constrained to lie in a known bounded convex polytope. We present a controller that is globally stabilizing and gives continuously improving bounds on the worst-case state deviation. The proposed controller simultaneously learns the system parameters and controls the system. The controller emerges naturally from an optimization problem and balances exploration and exploitation in such a way that it is able to efficiently stabilize unstable and adversarial system dynamics. Specifically, if the controller is faced with large uncertainty, the initial focus is on exploration, retrieving information about the system by applying state-feedback controllers with varying gains and signs. In a pre-specified bounded region around the origin, our control strategy can be seen as passive in the sense that it learns very little information. Only once the noise and/or system parameters act in an adversarial way, leading to the state exiting the aforementioned region for more than one time-step, our proposed controller behaves aggressively in that it is guaranteed to learn enough about the system to subsequently robustly stabilize it. We end by demonstrating the efficiency of our methods via numerical simulations.