Laser Doppler Vibrometry (LDV) is an established tool in automotive engineering for non-contact vibration measurements. The LDV method is not just limited to measurements on single test points but can also be used for full field scanning of entire structures, such as vehicles, braking systems, etc. Such Scanning LDVs (SLDVs) have found many applications in the automotive industry. However, for full body modal analysis, the use of SLDVs has traditionally been restricted due to the fact that only the Out-of-Plane (OOP) component of the velocity vector could previously be measured, whereas tri-axial accelerometers provide OOP and In-Plane (IP) data simultaneously. This limitation has been overcome with full 3-dimensional scanning LDVs, since 2002. A 3-D SLDV consists of three independent SLDV measurement heads, each measuring the vibration of a scan point from a different direction. The working principle and mathematics behind are discussed later in this paper.
A prototype production 3-D SLDV was developed in close cooperation with the brake NVH division of Bosch. Consequently brake noise measurements became the first application of 3-D SLDVs and this new technique contributed significantly to the understanding of brake dynamics, such as the coupling of IP and OOP modes. 3-D measurements on brakes are briefly discussed in this paper.
Another recent innovation has been the combining of 3-D SLDV with an optical geometry measurement technique. This facilitates and enables the relative spatial location of the SLDV sensors and the geometry of the test object to be accurately established, without needing prior knowledge of the shape of the object and/or Finite Element (FE) data to define the measurement points location.
Vibration measurements on a car body in conjunction with measured geometry data are discussed in this paper. Finally an outlook is given to future development goals in 3-D SLDV measurement techniques.