Advanced modal testing using continous scanning LDV methods

Modal testing is a technique used to measure Frequency Response Functions (FRFs) of test structures so as to determine the modal properties. Measurements of vibration can be performed using transducers which can measure the response of a specimen either by contact or non-contact methods. Accelerometers, a contact type, are the most common used in modal testing. Nowadays measurement systems, such as Scanning Laser Doppler Vibrometer (SLDV) one, are becoming more used in modal testing practice thanks to the ability of measuring vibration remotely. It is still a common practice in some engineering applications to use such systems in replacement either of a set of accelerometers or a roving impact hammer. The measurement method, performed using a SLDV, consists of acquiring vibration responses by moving a laser spot onto pre-defined measurement positions marked on a test structure; this is called stepped scan mode. However, measurements of vibrating structures can also be performed by sweeping continuously, instead of stepping, a laser beam over the surface. This paper presents an advanced method for carrying out modal testing using Continuous Scanning LDV methods and multi-tonal excitation waveforms. The improvement of this novel technique relies on the storage of meaningful information in a single LDV time history and so the modal properties of test structures can be obtained more quickly.