Operational Modal Analysis of a Spacecraft Vibration Test

Abstract

CASSIOPE is a small Canadian spacecraft carrying two payloads: the e-POP payload consisting of eight scientific instruments and the CASCADE payload - a high speed Ka-Band communication technology demonstrator. As part of the Cassiope environmental test campaign, the spacecraft was subjected to sine vibration testing for structural qualification, and to random vibration testing for workmanship verification. The purpose of the spacecraft sine vibration test was twofold: subject the spacecraft to protoflight launch environment and to provide a test verified structural model for final coupled loads analysis with the launch vehicle. In this paper, we employ an operational modal analysis technique to the ground vibration sine qualification test of the Cassiope spacecraft. The purpose of this exercise was to determine the applicability of operational modal processing to vibration qualification tests in cases where interface forces between the vibration shaker and the test article are not able to be determined in the course of the test and traditional modal techniques are not applicable. This paper will examine the ability of operational modal techniques in separating complex vibration dynamics and modal interactions present in sine vibration qualification tests and in aiding the determination of required test notching. Cassiope (CASCADE Smallsat and Ionospheric Polar Explorer) is a Canadian smallsat scheduled to be launched in 2011[1]. The three goals of the Casssiope mission are to demonstrate the high speed Ka-Band store and forward capability of the CASCADE CX payload (provided by MacDonald, Dettwiler and Associates Ltd.) [2], to investigate the atmospheric and plasma flows and related wave particle interaction and radio wave propagation in the topside ionosphere with a suite of eight e-POP (Enhanced Polar Outflow Probe) science instruments provided by the University of Calgary, and to develop a Canadian Smallsat Bus (provided by Bristol Aerospace Ltd). Development programs for space hardware generally envision two phases of testing. The first stage would be a modal survey test consisting of excitation by multiple small shakers and optimized response channels throughout the spacecraft. The purpose of this test is to allow updating of the finite element models to allow for better estimates of the flight loads through coupled loads analysis and also better predictions of the vibration test response levels and estimation of notching profiles. However, in many cases, due to budgetary or schedule constraints, the modal test is often omitted. The second stage of testing is qualification testing consisting of sine vibration testing, random vibration testing and acoustic testing. The sine vibration testing is closed loop controlled over a limited frequency band, with notching appropriate for the loads predicted during launch (these may be base shear or bending moment, c.g. acceleration, etc). The random vibration testing is often called for as a workmanship test or to capture low frequency content not easily excited by an acoustic test. This paper first looks at an overview of the Cassiope spacecraft. This is followed by an outline of the vibration test setup and sequence. The next section looks at the use of operational modal technique in processing the sine vibration test data. The modes in each axis are determined and their interactions are discussed. The applicability of the modal results to aid in the prediction of sine test notching is presented.