The measurement of the absolute displacement of a noisy disc brake

The investigation of in-plane vibration of a noisy disc brake is problematic because it is difficult both to measure and to verify. Because of the disc structure and the inability to visualize disc in-plane vibration, there has been reluctance by researchers to accept the contribution of a displacement parallel to the surface of the object, or in-plane displacement, to noise generation. In addition to measuring absolute displacement, it has been difficult to isolate the in-plane and out-of-plane components of displacement using either non-contact or conventional displacement measurement techniques. This paper investigates absolute displacement of a brake disc during noise generation. Double-pulsed holographic interferometry is used to record a series of time-related images of the brake head from three different angles of observation. Because each image views the brake head from a different perspective, each of them records a different degree of in-plane and out-of-plane displacement. By careful analysis of the three images, it is possible to isolate the in-plane displacement from the out-of-plane displacement. The time-related series allows the displacement to be investigated over a full cycle of excitation and so create an animation of the mode of vibration. It is seen that the in-plane displacement is complex and that its amplitude may be about twice that of the out-of-plane displacement.