Six degrees of freedom measurement with continuous scanning laser doppler vibrometer

The dynamic response of a structure can consist of up to six degrees of freedom (DOF). Not all of them may be present at all times in a structure, but for a detailed finite element model validation it is important to provide as much information as possible. Current contact transducers can pick up a combination of some of the mentioned DOFs, but are not able to provide all six DOFs from a single transducer. Several contact-less transducers, such as a Scanning Laser Doppler Vibrometer (SLDV), can be combined to a single measurement system that allows the determination of up to three translational degrees of freedom at a point, but they are still not providing rotational information as the laser beam is fixed to a point. Continuously Scanning Laser Doppler Vibrometer (CSLDV) methods can partly overcome this issue by travelling in small circles around the measurement location and thus also sensing the rotation around the three axes at a point. An introduction of CSLDV methods for measuring DOFs at a point is given, focussing on a method for capturing up to 5 DOFs using a lens. However, the lens can make the measurement of the in-plane rotation very difficult and impractical. This paper presents a new type of scanning device which allows measuring all six degrees of freedom at a measurement location with a single setup and a single point LDV, by combining variable small circle scans. Experimental validation of the scanning head is presented and the measurement of the last DOF, the in-plane rotation, is demonstrated.