UIO(unknown input observer) is frequently used to design FDI(fault detection and isolation) filter. Patton et al in 1998 gave the necessary and sufficient condition for the existence of FDI filter but it is not quite satisfactory in actual use. We now present a more useful necessary condition for its existence.rn Our necessary condition comes naturally from our design of UIO. In section 1, we give eqs.(1a) through (15b) for designing UIO on the basis of singular value de composition. Eq.(25) gives the necessary condition in terms of the original system matrices.rn Simulation results given in Figs.1 and 2 show preliminarily that our necessary condition does guarantee the existence of FDI filter and that eqs.(18) through ( 24) can make the design of FDI filter effective.%利用奇异值分解技术，构造一种未知输入观测器，并在此基础上设计FDI滤波器，实现 控制系统的故障检测与隔离。由原始系统矩阵，给出了FDI滤波器存在的必要条件。仿真算 例验证了所提方案的有效性。
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