On the design of unknown input observers and fault detection filters

This paper examines the design of unknown input observers from a frequency domain point view and shows that a UIO design can be formulated as a perfect H or H disturbance rejection problem that many tools in robust control field can be applied. In particular, it is shown that a more general nonperfect disturbance rejection or approximate UIO problem may be considered and potentially interesting open problems are outlined. The paper also establishes the connection between residual error based on the UIO fault detection filter and residual error based on a standard observer. It is shown that a UIO based filter may be useless for fault detection. Hence it is critical to find the suitable UIO filters from the family of all possible UIO filters to fit the specific fault detection needs.