Rotational degrees of freedom : an historical overview on techniques and methods

Most of the tools used in the analysis of vibro-acoustic systems are based on the manipulation of frequency response function (FRF) matrices. The different procedures foresee the use of complete matrices where both translational and rotational degrees of freedom (TDOF and RDOF) information are contained. Unfortunately the second group of data is very often neglected with substantial errors in the final results. In order to overcome this problem, known since the early sixties, many solutions have been tried using existing sensors, developing new ones, defining different methodologies and approximations. This article will give a short review of the different techniques used to measure and excite accurately the RDOF data. Following a historical path, the different procedures will be presented, the finite difference methods, the t-block approaches, the multi-point approximations, the single beam LDV, the multiple laser systems, the scanning laser procedure, the magnetostrictive moment shaker, the pure moment exciter and so on. This work will hopefully help the researchers involved in dynamic modelling to select the most appropriate tools and procedures