Indirect input identification by modal filter technique

Abstract

This paper is a study on modal model method in estimating system inputs from vibration responses, which is one of the indirect input identification methods in frequency domain. The method has advantages over the direct inverse method especially when points of operational inputs are inaccessible for artificial excitation but accessible for response measurement. Systematic procedures of an extended modal model method are proposed and checked by numerical experiments. Mechanisms of error propagation, i.e. how errors in modal parameters such as poles and mode shape vectors affect estimation of the input forces, are illustrated. Then, in order to counteract the error propagation, discrete modal filter approach is used in this paper to compute the inversion of modal matrix in which most serious errors seem to be generated. Further, a reduced form of modified reciprocal modal vector (MRMV) is proposed to estimate multiple inputs, which is shown to have orthogonality errors smaller than by the MRMV filter. (C) 1999 Academic Press.