Laser Doppler Vibrometry (LDV) is an established tool in automotive engineering for non-contact vibration measurements. The LDV method is not limited to measurements on single test points. Complete structures can be tested by utilizing a pair of galvano-electrical mirrors for steering the laser beam in x and y directions across the surface. Such scanning LDVs (SLDV) have found many applications in the automotive industry. However for full body modal analysis use of scanning LDVs has so far been restricted due to the fact that only the out-of-plane (OOP) component of the velocity vector can be measured whereas tri-axial accelerometers provide OOP and in-plane data simultaneously.
This limitation has been overcome with three-dimensional scanning LDVs which were first introduced towards the end of 2002. A 3D SLDV consists of three independent SLDV measurement heads, each measuring the vibration of a scan point from a different direction. In order to perform a measurement, all three laser beams are positioned simultaneously onto the measurement point. Knowing the vector of each beam, the vibration data can be converted into the orthogonal coordinate system of the measured object. The working principle and mathematics are discussed hereafter.
For performing 3D measurements, the position and orientation of the scan heads (beam vector) in a pre-defined coordinate system have to be determined prior to scanning. To facilitate this task, the SLDV technique is combined with an optical geometry measurement technique. This combination has resulted in easy to apply methods for determining the scan head position within the coordinate system of the test object without prior knowledge of the geometries of the object.
A prototype of the 3D SLDV was developed in close cooperation with the brake NVH division of Bosch, Germany. Consequently, brake noise measurements became the first application of 3D SLDVs and this new technique contributed significantly to understanding the mechanisms such as the coupling of in-plane and out-of-plane modes. Three-dimensional measurements on brakes are briefly discussed in the application section of this paper.
Recently, merging, or “stitching” of different measurement sets into one global result file has been made possible. The main focus of this paper is consequently set on vibration measurements on full car bodies in conjunction with measured geometry data. Merging the measurements taken from different directions results in a 3D model which can be directly compared to FE data. Several full car body measurements are presented and discussed in this paper, as well as a detailed full body measurement on a cylindrical cover of an electrical motor. Much of the theory for 3D Scanning Laser Doppler Vibrometers has been presented in an earlier paper this year [1].