Frequency response sensitivity functions for helicopter frequency domain system identification (Reprinted from Journal of the American Helicopter Society, vol 42, pg 244-253, 1997)

This paper addresses some aspects of frequency domain based, state space sy stem identification, A frequency response sensitivity function is developed , and applied to the problem of identifying a coupled rotor-fuselage helico pter model from simulated frequency sweep data This function indicates how sensitive the frequency response of the model is, at every frequency of int erest to perturbations of ore model parameters. This information can be use d to tailor the identification process by indicating what frequency bands a nd what inputs are best suited for the identification of a given parameter. The paper also presents expressions for an efficient calculation of the de rivatives of this sensitivity function. This enables faster implementations or the frequency response matching problem and of the calculation of accur acy metrics, The sensitivity functions are used to partition an identificat ion problem that covers a very wide frequency band into two identifications over smaller frequency bands, Such frequency-banded identification appears possible, faster, and slightly more accurate for the case studied in the p aper. A possible application of this technique is the identification of a c oupled rotor-fuselage system from separate low frequency flight tests and h igher frequency, rotor wind tunnel tests. The results indicate that the use of the frequency response sensitivity function can help automate some deci sions typically left to the analyst's judgment.