State feedback control assumes that all of the state power can be observed. However, all of the state power cannot be observed in fact. As a solution in such a case, output feedback control of performing feedback control directly without using an observer from the output of the system detected by the sensor is proposed. Considering an actual system, various noises mix in the observation process by a sensor, and when controlling, it is thought that it has a certain influence. However, about the proposed output feedback control, observation noise is not included in the system. In this paper, we consider a robustness of a certain kind over the observation noise of output feedback control, and we show a means by which a upper bound of the power spectral density of observation noise evaluates the influence of the observation noise which is known.
