Measuring rotational degrees of freedom using a laser Doppler vibrometer

Many model updating and dynamic coupling techniques require the response of a structure to be defined at ail degrees of freedom. Standard experimental techniques do not routinely allow the measurement of rotational DoFs. Also time constraints do not permit measurement over a dense grid. A laser syst em has been developed which enables rotations to be Extracted by a simple p lane-fitting technique, which is described in this paper. A finite element model-based parametric study is presented, which has shown that the perform ance of the technique is dependent on the amount of corruption present on t ranslation data. A semi-empirical technique is developed, using second derivatives to exacer bate temporarily the noise corruption, which quantifies accurately the equi valent Gaussian noise on the response data. An experimental study is also p resented which shows the considerable promise of these procedures. Finally, a brief description of a model updating case study is presented for illust rative purposes. Terminology. In order to avoid a verbose sentence construction, the coordinate at which it is desired to measure the rotational frequency response functions (FRFs) is hereafter referred to as the rotation point. In addition, the distance between the,measured translation FRFs and the rotation point is referred to as the measurement radius.