Filtering and parametrization issues in feedback rel

This paper discusses ltering and parametrization issues involved in the usage of fractional representations in multivariable approximate and feedback relevant identi cation of a possibly unstable plant operating under closed loop conditions The knowledge of the controller is used to access any stable right coprime factorization of the plant by measuring and ltering the signals present in the closed loop system By exploiting a spe ci c class of parametrizations in the estimation of the stable coprime factorization with a prespeci ed McMillan degree a linear time invariant model having the same McMillan degree will be obtained In addition the approximate and feedback relevant estimation of a xed order linear time invariant model based on coprime factor identi cation leads to an additional constraint which can be written down explicitly as a relation between the ltering of the signals present in the closed loop system and the coprime factors of the model being estimated A possible solution to deal with this constraint based on an update algorithm is presented