  In this paper, we review some recent results about the use of dynamic
observers for fault diagnosis of discrete event systems. Fault diagnosis
consists in synthesizing a diagnoser that observes a given plant and identifies
faults in the plant as soon as possible after their occurrence. Existing
literature on this problem has considered the case of fixed static observers,
where the set of observable events is fixed and does not change during
execution of the system. In this paper, we consider dynamic observers: an
observer can "switch" sensors on or off, thus dynamically changing the set of
events it wishes to observe. It is known that checking diagnosability (i.e.,
whether a given observer is capable of identifying faults) can be solved in
polynomial time for static observers, and we show that the same is true for
dynamic ones. We also solve the problem of dynamic observers' synthesis and
prove that a most permissive observer can be computed in doubly exponential
time, using a game-theoretic approach. We further investigate optimization
problems for dynamic observers and define a notion of cost of an observer.
