Safe Stability Envelopes and Shared Control for Active Vehicle Safety

With advances in vehicle modeling and control, intelligent vehicles can increasingly utilize the full vehicle capabilities should it be necessary for safety. This paper presents a shared control approach capable of operating in the open-loop unstable state space of a vehicle. First the Maximal Phase Recoverable Parallelogram is defined as the state space where a control action exists that can stabilize the vehicle. An online approach to constructing a subset of the Maximal Phase Recoverable Parallelogram that avoids spinning out past a desired sideslip is presented. This then forms a safety envelope for a Nonlinear Model Predictive Control framework for shared control. The shared control formulation is designed to match driver inputs, unless inputs lead to safety violations such as a spin or track bound violation. If intervention is needed, the controller seeks to follow the drivers intent in a safe manner. Results for a full scale experimental vehicle executing circular steady-state drifting demonstrate the ability of the controller to operate in the open-loop unstable drifting regime and limit intervention unless there is a risk of spinning out or violating track bounds. This shared control approach is a step towards assisting a driver in using a vehicle’s full capabilities even in extreme maneuvers.