Abstract: A full order PI observer is proposed for linear continuous systems with unknown inputs appearing in both process and measurement, which is an extension of the PI observers for systems with only unknown plant disturbance. The PI observer has two feedback loops, a proportional loop and an integral loop of the estimation error; in this way, the PI observer can estimate not only the states but also the unknown inputs. The structure and the estimation performance of the PI observer are discussed and analyzed, and the conditions are given and proved. Owing to the weaker assumptions than UIO (Unknown Input Observers), it can be applied by more observer based technical solutions. The design method is declared and carried out with illustrative examples of the linearized dynamics of the L 1011 aircraft. The comparison between estimation performance of the PI observer and Luenberger observer shows the better performance of the former.

Key words: unknow n input, unknow n inputs observer, p ropo rt ional integral observer