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BSC and HAM Chamber Structural Modifications



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LASER INTERFEROMETER GRAVITATIONAL WAVE OBSERVATORY LIGO Laboratory LIGO LIGO T030073 01 12 APRIL 2003 BSC and HAM Chamber Structural Modifications Considered for the External Pre Isolator EPI System D Coyne J Kern B Lantz K Mason Distribution of this document HEPI SEI Group This is an internal working note of the LIGO Project California Institute of Technology LIGO Project MS 18 34 1200 E California Blvd Pasadena CA 91125 Phone 626 395 2129 Fax 626 304 9834 E mail info ligo caltech edu Massachusetts Institute of Technology LIGO Project NW17 161 175 Albany St Cambridge MA 02139 Phone 617 253 4824 Fax 617 253 7014 E mail info ligo mit edu LIGO Hanford Observatory P O Box 1970 Mail Stop S9 02 Richland WA 99352 Phone 509 372 8106 Fax 509 372 8137 LIGO Livingston Observatory P O Box 940 Livingston LA 70754 Phone 225 686 3100 Fax 225 686 7189 Advanced LIGO LIGO T030073 01 D 1 Introduction During assembly and testing of the external pre isolators at LASTI we found low frequency excitations caused by the HAM crossbeams and the BSC piers We are investigating two approaches to dealing with these problem resonances stiffening of the structures and various damping techniques Modal analysis using Algor s finite element software is used to model the structural modifications Accelerometers placed along the structure and the measured transfer functions are used to evaluate the effect of the changes 2 HAM Crossbeam Modifications A modal survey was conducted on the MEPI HAM system before structural modifications were undertaken The results can be found in L Zuo O Rifai S Nayfeh Modal Analysis and Feedback Control of HAM MEPI LIGO G030187 00 The low stiffness of the HAM crossbeams created challenges to the active control system due to low frequency high Q resonances in and near the control band The lower modes of the system were not rigid body like in character but instead had significant elastic participation Initial attempts at closing loops were difficult due to several large



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