MOLA Force Limiting and Notching for Random Vibration Testing

There are five steps to notching. They are:

  1. Perform a NASTRAN random vibration analysis run using the initial random input spec;
  2. Determine the maximum allowable force (i.e., the limit) for the instrument -- this is usually a curve in lb2/Hz over a range of frequencies;
  3. Apply the limit to the initial response to determine the frequencies where notching will occur;
  4. Divide the limit by the response at a given frequency for the limiting factor if the response is greater than the limit; otherwise, the limiting factor is 1.0;
  5. Multiply the limiting factor times the input spec at the same frequency; do this for every frequency where the response is above the limit.
Step two is often performed by the people doing force limiting calculations. This is the case with MOLA. Engineers at the Jet Propulsion Laboratory waved their magic wand (i.e., it's beyond the scope of these pages) and created the upper limit for MOLA while preparing their data that they will use during random vibration testing using force limiting. Otherwise, you or your project will need to create your own upper limit.

This is all rather confusing, but going to the results should clear that up. Following are the random responses in each of the three axes with the limits drawn in, and the resulting notched input spec. You may want to refer to the original input specs for a comparison (use the protoflight input).

Back to the MOLA Random Vibration Page.
Back to the MOLA Mechanical Analysis Page.
Goddard MeatballThis page is maintained by Ryan Simmons, at Ryan.Simmons@nasa.gov.
This page was last updated on January 18, 1996.