Positive systems

Positive systems constitute a class of systems that has the important property that its state variables are never negative, given a positive initial state. These systems appear frequently in practical applications, as these variables represent physical quantities, with positive sign (levels, heights, concentrations, etc.).

The fact that a system is positive has important implications in the control system design, as the system. It is also important to take this positivity into account for state observer design, as standard observers (for example Luenberger observers) might give illogical negative values.