Fault tolerant control for unstable systems: a linear time varying approach

In (passive) fault tolerant control design, the objective is to find a fixed compensator, which maintains a suitable performance - or at least stability - in the event that a fault should occur. A major theoretical obstacle to obtain this objective, is that even if the system models corresponding to the occurrence of various faults are simultaneously stabilizable by a linear, time-invariant compensator, this compensator might have to be of a very high order, as shown in a recent publication. In this paper, we propose a design procedure for a time-varying compensator, which overcomes the obstacle for any finite number of faults with a controller order of no more than the plant order. The performance of this compensator might be poor, but a heuristic procedure for improving the performance is also shown, and an example demonstrates that this improvement can be truly significant.