FREQUENCY-RESPONSE SENSITIVITY FUNCTIONS FOR HELICOPTER FREQUENCY-DOMAIN SYSTEM-IDENTIFICATION

This paper addresses some aspects of frequency domain based, state spa ce system identification, A frequency response sensitivity function is developed, and applied to the problem of identifying a coupled rotor- fuselage helicopter model from simulated frequency sweep data, This fu nction indicates how sensitive the frequency response of the model is, at every frequency of interest, to perturbations of the model paramet ers, This information can be used to tailor the identification process by indicating what frequency bands and what inputs are best suited fo r the identification of a given parameter, The paper also presents exp ressions for an efficient calculation of the derivatives of this sensi tivity function, This enables faster implementations of the frequency response matching problem and of the calculation of accuracy metrics. The sensitivity functions are used to partition an identification prob lem that covers a very wide frequency band into two identifications ov er smaller frequency bands, Such frequency-banded identification appea rs possible, faster, and slightly more accurate for the case studied i n the paper, A possible application of this technique. is the identifi cation of a coupled rotor-fuselage system from separate low frequency flight tests and higher frequency, rotor wind tunnel tests, The result s indicate that the use of the frequency response sensitivity function can help automate some decisions typically left to the analyst's judg ment.