Force Limited Vibration Testing Applied to the JWST FGS OA

Abstract

The Fine Guidance Sensor (FGS) is the Canadian contribution to the James Webb Space Telescope (JWST). The optical assembly (OA) is one of the components of the FGS. In October 2009, the Engineering Test Unit (ETU) of the FGS OA was tested in preparation for testing of the flight hardware planned for the Spring of 2011. The presence of a required interface ring having about 25% of the test item mass (80 kg) complicated the planning and performance of the Force Limited Vibration (FLV) testing. Modifications to our standard FLV procedure were developed and successfully applied during testing of the ETU. Such modifications were deemed necessary based on pre-test analysis showing that the FGS OA apparent or dynamic mass was significantly changed due to the presence of the interface ring. The paper first main part presents results of the pre-test analysis that demonstrated that the proposed simple modifications to the standard FLV procedure would ensure conservatism during testing (when compared to testing without the interface ring). The other main part of the paper presents details and results of the successful ETU vibration test. For one of the lateral axes, force limiting was combined with response limiting applied in the higher frequency range. The use of force limiting resulted in significant reduction of overtesting for all three axes of excitation. For the two lateral axes, the input acceleration PSD was notched by more than 20 dB; for the vertical axis, it was notched by 17 dB.