The out-of-plane vibration characteristics of a noisy brake are generally better understood than in-plane characteristics. The fundamental reason for this is that in-plane vibration was not considered a significant effect until recently when technology has allowed the in-plane vibration characteristics to be determined with some degree of confidence. Detailed investigations of the side views of out-of-plane holographic images indicated that the in-plane displacement could be quite significant and possibly larger than the out-of-plane displacement. It was because the fringe pattern could not be attributed solely to out-of-plane displacement that a study of in-plane vibration was initiated.
The paper discusses the measurement of both out-of-plane and in-plane vibration of a twin calliper disc brake during noise generation. The technique makes use of a series of 11 time related holograms recorded from three different viewing perspectives of the brake - a total of 33 holograms covering a single cycle of excitation.
Each image records absolute displacement but as each of the three holograms view the brake from a different viewpoint then each comprises varying degrees of out-of-plane and in-plane vibration, dependant on their angular position. This varying degree of displacement allows their manipulation to identify and isolate the in-plane and out-of-plane contributions to the overall excitation so that each may be considered separately. The out-of-plane is represented as a three-dimensional image whereas arrows represent the in-plane amplitude as a “quiver” plot.
The series allows a dynamic representation of each mode of vibration to be created separately and then combined to indicate the absolute vibration mode characteristics.
The results indicate the disc modal behaviour to be extremely complex for the two-calliper brake, the disc antinodes tending to “index” between the two callipers.