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Unformatted text preview: 40 EXPERIMENTAL TECHNIQUES January/February 2005 TECHNIQUES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . TECHNIQUES by R.M. French FABRICATING MIRRORS FOR LASER VIBROMETER TESTING Scanning Laser Vibrometer Drum Friction Surface Conical Mirror Fig. 2: LDV scanning brake drum with conical mirror L aser vibrometers are now common enough that they appear regularly in both the technical literature and in industrial test labs. Scanning laser vibrometers are best applied in situations where the test specimen is roughly planar and where the test surface is completely vis- ible from a single observation point. When this condition is not possible, mirrors can be used to view otherwise inacces- sible portions of the parts. Figure 1 shows a mirror array being used to scan the outer surface of a brake drum. However, a more elegant test setup is possible if the user can fabricate special mirrors as necessary. It is typical in optical applications to use high quality, front-surface mir- rors. Flatness of these mirrors ranges from a few wave- lengths of visible light down to 1/10 wavelength. Mirrors of this quality are not easily made without specialized and ex- pensive equipment. However, laser vibrometers do not nec- essarily need mirrors so precisely made. Indeed, this paper Fig. 1: Mirror array being used for LDV scan of brake drum describes a successful effort to man- ufacture conical mirrors from alu- minum billet. LDV Test of Brake Drums This work was motivated by the de- sire to use a scanning laser vibro- meter 1 to measure the dynamic structural response of an automotive brake drum. Initial testing used a commercially available conical mir- ror approximately 10 mm in diame- ter. As shown in Fig. 2, the mirror was placed in the center of the drum...
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This note was uploaded on 02/21/2012 for the course MET 311 taught by Professor Markfrench during the Spring '12 term at Purdue University-West Lafayette.
- Spring '12