Effects of cross-axis sensitivity and misalignment on the response of mechanical-optical accelerographs

Abstract

Recorded strong-motion accelerograms may be affected by cross-axis sensitivity and the misalignment of their transducer sensitivity axes. The cross-axis sensitivity arises from the fact that the transducer mass when deflected from its ideal equilibrium position may experience additional deflection caused by the inertial forces perpendicular to the transducer's principal sensitivity axis. The misalignment effects result from imperfect positioning of transducers into the instrument housing so that the nominal sensitivity directions are not mutually perpendicular and/or do not coincide with the orthogonal reference frame attached to the instrument housing. If the misalignment effects cannot be accounted for, the recorded accelerations can be distorted by an amount, which for large input accelerations, may exceed the digitization noise level by as much as one order of magnitude. In this paper a method is presented for a complete dynamic correction procedure which can eliminate any cross-axis and misalignment effects and yield exact accelerations for a reference coordinate system conveniently attached to the instrument housing. To carry out these correction procedures it is necessary to know the precise orientations of all transducers relative to the selected reference frame. A simple sequence of static tilt tests is proposed which provides adequate information for computation of all misalignment angles and estimation of transducer sensitivity constants.