1. General

Whether for monitoring or controlling a system, it is of paramount importance to have as much information as possible about the system's characteristic variables throughout its operation. These variables may be internal variables or causes which can influence system behavior without being measured. This can be achieved by using physical sensors to instrument the system [R 401] . However, in some situations, this is not always possible, and implementing a software sensor or observer is of prime importance.

As soon as the notion of state was introduced to model a system