Rotational Vibration Test Apparatus for Laser Vibrometer Verification

Prior to making rotational vibration measurements with a laser vibrometer, it is good practice to establish that the instrument is operating properly. This can be accomplished by comparative measurement of a rotational vibration source with known amplitude and frequency. This paper describes the design and development of a rotational vibration apparatus with known amplitude and frequency to be used as a reference for comparison to concurrent and co-located measurements made by a rotational laser vibrometer (RLV). The comparative measurements acquired with the apparatus are helpful to verify proper laser vibrometer operation in between regular calibration intervals, and/or whenever the functionality of the vibrometer is suspect. In the subject apparatus, a Cardan shaft with variable input speed and angle is used to provide output torsional vibration with variable frequency and amplitude. Previously derived equations of motion for Cardan joints are used to estimate operating amplitude versus speed relationship for the apparatus. To provide an independent rotational vibration measurement for comparison to the laser, a ferromagnetic toothed wheel and magnetic pickup are used. The design of the Cardan shaft and adjustable support table are described, followed by the design of the variable speed electric drive system and controller. For the finished apparatus, example operating data from the reference sensor are compared to laser vibrometer data, and to the theoretical prediction of output torsional velocity from the Cardan joint equations of motion. Finally, suggestions for future work are provided. The development and verification of rotational tuned vibration absorbers is provided as a potential additional application of the apparatus.