Investigation of data from disc brakes generating noise using laser holography

Considerable knowledge of the mode of vibration of the disc, pads and other components of real disc brake systems generating noise has been obtained using the technique of double pulsed laser holography. The technique has been developed further by using electronic triggering devices which enable a series of time related holographic images to be obtained, corresponding to different times in the cycle of vibration of the braking system.
By digitising the holographic images it has been possible, using image processing techniques, to develop algorithms that determine the co-ordinates of the hologram fringe lines on the exposed portions of the disc and pad surfaces. From this information three-dimensional representations of the vibrating disc and finger pad surfaces can be constructed. Furthermore, from the series of time related images, a three dimensional animation of the real motion of the disc and pad surfaces can be obtained.
Using least-squares mathematical approximations it is also possible to generate accurate Fourier Series type representations of the vibrating disc and pad surfaces, showing both in-plane and out-of-plane motion. Holograms of the disc rim show how the vibrating motion varies throughout the thickness of the disc. It is believed that this investigation will be important in developing mathematical models of disc brake squeal.