Abstract: With the rapid advancement of today's ultra-highperformance mechanical or mechatronic system such asmagnetic or optical disk drives, improving metrologycapabilities to examine the performance characteristicsof these system are growing ever more important. Theprimary tested studied in this paper is an ultra-highprecision ball-bearing spindle that possessesnon-repeatable runout of less than 100nm. The metrologytool adopted is laser Doppler interferometer systemthat has Megahertz bandwidth and nanometer resolutions.Experimental data obtained clearly indicates thatmeasuring vertical runout of a spindle motor is astraightforward process. However, a fundamental effectwas identified, where the radial runout data was foundto drift upward or downward with time, when using thelaser Doppler system to measure the radial runout ofultra-high precision rotational systems whose surfaceprofile is not flat. All of the underlying reasons thatcause this undesirable effect were proposed andverified. Approaches that can be adopted to circumventthis apparent limitation on adopting the laser Dopplerinterferometer systems to measure rotational curvedsurface were implemented to further extend itsapplication horizon. The experimental data realized andthe application experience obtained were shown tofurther advance our measurement capabilities. !6
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