Dynamic Measurements on a Star Tracker Prototype of AMS Using Fiber Optic Sensors

An aluminum prototype of the Amica Star Tracker Support (ASTS) of the AMS_02 (Alpha Magnetic Spectrometer) experiment has been instrumented with two Fiber Bragg Gratings (FBGs). AMS_02 is a 7 tons particle physics experiment that is planned to run on the International Space Station for 3 years starting from October 2005. The star tracker, as all the sub-detectors in AMS, has to satisfy the structural verification plan requirements. In the qualification process of the star tracker FBG sensors may be used to measure static and dynamic strain in the structure during the tests. FBGs are basically optical strain gages that offer several advantages over the conventional ones such as immunity from electromagnetic interferences, small dimensions, low specific weight and the possibility of multiplexing many of them along the same fiber. In this work the use of FBGs in the acquisition of strains during a dynamic test on the ASTS semplified prototype is reported. The excitation has been provided by an instrumented impact hammer while the response has been given by one FBG and one accelerometer nominally in the same location. All the time histories have been recorded and transformed in the frequency domain to retrieve the Frequency Response Functions (FRFs) and the Strain Frequency Response Functions (SFRFs), both providing immediately the resonant frequencies of the structure. The contemporary use of the two sensors with many excitation points allow to retrieve accurate displacement and strain mode shapes with the relative resonant frequencies. As future work we plan to use a higher number of sensors in different locations and orientations.