Graceful Safety Control: Motivation and Application to Battery Thermal Runaway

This paper surveys the literature on barrier function -based safety control and identifies a critical research gap, namely, the need for safety control algorithms that degrade gracefully in the presence of system failures and anomalies. The paper proposes a novel control design paradigm that embeds the notion of graceful degradation within control barrier function theory. Intuitively, the idea is to switch from one safety control mode to another in response to changes in the system's environment and/or damage/hazard state. Mathematically, we achieve this by constructing a non -monotonic barrier function, then utilizing slack variables to "soften" the resulting safety constraints. We illustrate this approach for a simple model of the thermal runaway dynamics of two cells in a battery pack. When one battery cell experiences thermal runaway, a benchmark safety controller focuses its cooling efforts on that hot cell, even when doing so is futile. The proposed controller, in contrast, focuses on preventing thermal runaway propagation instead. The end result is quite profound: both battery cells burn down in the benchmark case, but only one cell burns down with the proposed graceful safety controller.