The measurement of brake squeal vibrations is a difficult problem, because the rotating disc does not allow any physical contact with the sample during measurement. The use of accelerometers is difficult and very time consuming. Non-contact scanning techniques (e.g. vibrometers) are widely used for these applications, and although they provide a very high temporal resolution is available, still problems occur if non steady vibrations are present. Furthermore, the recent interest of researchers has focused not only on the one-dimensional out-of plane deformation, but on the determination of the three-dimensional vibration behavior, especially the analysis of the in-plane component of the vibration.
Electronic speckle pattern interferometry (ESPI) using pulsed lasers has already been presented as a non-contact and full field measuring technique of brake squeal, which provides complete maps of the vibration of any component of the brake during test. Brake discs, drums, calipers, pads have been measured with this technique. The component under investigation -a rotating brake disc with engaged caliper - is illuminated by two very short laser pulses with a predetermined time separation. The images at these two laser illuminations are recorded with a special high speed camera and analyzed in a computer. The calculated result is a deformation map of each point of the brake disc and caliper which describes the movement of the individual points of the brake between these two laser pulses. Therefore, this technique literally freezes two moments of the movement of the brake and gives an instantaneous view of the overall deformation. Recent developments have produced 3D systems, which provide the complete three-dimensional vibration map of the brake. Special algorithms are used in order to separate the rotation component of the brake disc from the in-plane vibrations.
Using this technique completely automatic brake squeal inspection systems have been designed, which capture the squealing signal, automatically fire the laser and provide the complete deformation map of the component under test. In this paper, we describe this promising new technique and give examples of recent applications in the field of brake squeal analysis.