Unbiased parameter estimation of linear systems operating under linear feedback

Identification of dynamic systems operating under feedback control is an important issue. A bias correction method was previously proposed for indirect identification of closed-loop systems with disturbances. A key assumption used in deriving this method required that the order of the controller is not less than that of the open-loop plant. In this paper it is shown that this assumption can be removed by prefiltering the input data through a digital prefilter, so that this method can be applied to unbiased identification of closed-loop systems with low order controllers.