Safe-Parking Framework for Fault-Tolerant Control of Transport−Reaction Processes

This work considers the problem of handling actuator faults in transport−reaction processes described by quasi-linear parabolic partial differential equations (PDEs) subject to input constraints. To this end, first, by exploiting the separation between the fast and slow eigenmodes of the parabolic spatial differential operator in combination with Galerkin’s method, a finite set of ordinary differential equations (ODEs) that captures the dominant dynamics of the PDE system are constructed. This finite ODE system is used to develop a Lyapunov-based model predictive controller which provides an explicit characterization of the set of initial conditions from where closed-loop stability of the parabolic PDE system is guaranteed. This control design is then subsequently used to develop a safe-parking framework for handling faults. In particular, faults which preclude the possibility of maintaining operation at the nominal equilibrium distribution, using the existing robust or reconfiguration-based fault-tolerant control approaches are considered. The key idea is to temporarily maintain the process at an appropriate “safe-park” distribution using the available depleted control action. This “safe-park” distribution is chosen to prevent onset of hazardous situations as well as to ensure smooth resumption of nominal operation upon fault repair. Utilizing the stability region characterization provided by the developed predictive controller, safe-park distributions from the safe-park candidates (equilibrium distributions subject to the remaining functioning and fail-safe values of the failed actuators) are chosen to preserve closed-loop stability upon fault repair. The proposed framework is illustrated on a diffusion−reaction process.