Enveloping Method of Generating Vibration Load Levels– An Analytical Case Study
- Paper number
IAC-07-C2.3.05
- Author
Dr. S.M. Veenaranjini, Indian Space Research Organisation (ISRO), ISAC, India
- Coauthor
- Coauthor
Gajbir Singh, India
- Coauthor
Prasanna Potdar, India
- Year
2007
- Abstract
In this paper, a case study is presented to quantitatively understand the possible perils of enveloping method of generation of dynamic load levels. A cascaded three-tier system typical of a spacecraft situation is considered for this purpose. Enveloping method, works on the basic assumption that when the response of the base of a subsystem is given as input the output is nearly same for all frequencies, damping and frequency ratios. Take a cascaded two degree–of- freedom system which is subject to excitation x. Let the individual natural frequencies of the two systems in the stack be f1 and f2. The response of m1 and m2 be x1 and x2 respectively. Now for testing second system separately if we take te response of m1 in the first case and apply it as input to the separated system output X2’ will closely approximate x2 It has been shown earlier in literature that it does provide good approximation except in certain conditions. One glaring example is that of a dynamic absorber situation, where f2=f1. When m1 forms a node and m2 shows very high amplification, which can never be captured by the second set up. Besides real life situations are more complex due some or more of the following reasons. 1. Each system exhibits large number of important modes in the frequency range of interest. 2. Interfaces are not single-points, but multi-point interfaces. 3. These multi point interfaces see differential responses, hence enveloping of responses will be necessary. 4. Damping of some modes can be significantly different at system and subsystem level. 5. Modes of the subsystem/systems can get altered significantly due to assembly. 6. Several levels of cascading is present 7. Testing level of the subsystem will have to encompass various loading cases at the system level. In order to get a quantitative feel of the errors that can be introduced in enveloping method a typical spacecraft is studied here. From the simulations it is conclusively seen that the second method can result in very large under-test at lower frequencies and very high over-test at high frequencies. The difference in testing reduces in systems with large damping. Particularly when systems have lower damping one has to be really careful while generating the specifications. It is best to carry out highest system-level simulations to arrive at the specifications. This study also highlights the need for input/output limitations
- Abstract document
- Manuscript document
IAC-07-C2.3.05.pdf (🔒 authorized access only).
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