If you are interested in learning some basics about frequency response analysis, then feel free to read on or make the jump listed below. If you are only interested in MOLA-specific information, then click on your area of interest in the list. If you have any questions, then you can contact me at the e-mail address located at the bottom of this page.
When frequency response analysis or testing is performed, an input spec is needed. The spec gives the frequency range, e.g., 5-100Hz, and it gives the magnitude, generally an acceleration.
In this set of pages you will find the response charts of the following MOLA components:
An exploded view of the model is available to identify the components.
In this set of pages you will find the force response chart of the MOLA instrument. This chart shows the force acting at the spacecraft/instrument interface that is caused by the sine vibration of the MOLA instrument FEM.
Unlike random vibration, sine vibration can be directly measured for any frequency, since only one frequency occurs at a time (see Random Vibration Analysis for details). Using analysis, we can determine accelerations of any point on a finite element model. That information is in turn used to determine loads on components or on the entire instrument.
Frequency response analysis uses the relationship between displacement and acceleration, i.e., acceleration is the second derivative of displacement with respect to time:
That is, the acceleration equals the amplitude times the frequency squared (where the frequency is in radians/second). If you know your frequency and amplitude, then you know your acceleration. (Thus, in the input tables, for lower frequencies, double amplitude is used instead of an acceleration.)
In sine vibe testing, accelerometers are attached to the test component and the accelerations are measured directly.
As with random vibration, notching of the input spec can also be used if there is an upper limit on what the instrument should encounter. In the case of sine vibration, finding that upper limit is generally much easier than in random vibration. This is because the responses are given as either acceleration vs. frequency or force vs. frequency. Therefore, we can apply an upper limit acceleration or force to the responses and where ever the response goes above the limit, we can notch the input spec at those frequencies.
In MOLA's case, we wanted to limit the acceleration that the instrument would encounter to 6g, or 6 times the weight of MOLA. These results can be reached by jumping to the MOLA Frequency Force Responses page.
Back to the MOLA Mechanical Analysis Page.
This page is maintained by Ryan Simmons, at Ryan.Simmons@nasa.gov.
This page was last updated on January 19, 1996.