Sine Sweep vs. Random for Pre and Post Testing

19_fuel-cell-vibration-test

Certain equipment must be designed and tested to withstand vibration.  This is common in the automotive, aerospace, military and other industries.    The equipment is typically mounted to a shaker table and then subjected to a base input random or sine sweep vibration test.   The random vibration is usually in the form of a power spectral density (PSD).

The sine sweep or random test level may represent a maximum expected field environmental, a parts and workmanship screen, or an envelope of both.   The level may also include a statistical uncertainty margin or a safety factor.

A common practice is to perform low-level sine sweep test before and after the full-level test in order to measure the transmissibility ratio and identify natural frequencies and damping ratios.   There must be at least one base input control accelerometer and one reference accelerometer for this test, where the reference accelerometer is mounted somewhere on the test item.   The before and after transmissibility curves are then compared to assess whether any of the response peaks have shifted in frequency or magnitude.   Any shift may indicate that some fasteners have loosened or some other change has occurred.  If so, further investigation is needed.  Ideally, two curves are identical such that no further evaluation is required.

Sine sweep is the traditional vibration test for the pre and post tests.  The purpose of this paper is to determine whether random vibration can be substituted for sine sweep, via an example.  This could be done for time saving.  Also, random vibration is easier to control than sine sweep.

A difference between sine sweep and random is that all modes are excited all the time for stationary broadband random.  There is only one excitation frequency at a given time in sine sweep vibration, and each mode will be excited individually if the modal frequencies are well-separated.  In addition, the random vibration used for shaker testing typically has a bell-shaped histogram curve, whereas sine sweep vibration with constant amplitude has a bathtub-shaped histogram.

Both sine sweep and random should give the same transmissibility results for a linear system per textbook theory, but there are some practical concerns for implementation of each.  The numerical example results will show that random vibration is adequate, although sine sweep remains the best choice because it can give finer resolution.

An example is given in:  sine_sweep_random_pre&post_test.pdf

See also:

Webinar Unit 3 Sine Sweep Vibration 

Beam Bending Natural Frequencies & Mode Shapes

– Tom Irvine

2 thoughts on “Sine Sweep vs. Random for Pre and Post Testing

  1. Hi Tom,

    Nice write-up. My experience has been predominantly with the random signature for pre-to-post comparisons. A 5% shift in frequency and a 10X on PSD or sqrt(10) on transmissibility are usually the industry accepted limits for comparison. It is true that the sine sweep is cleaner, but you may have challenges depending on the dynamic response characteristics of the item under test when mounted to the shaker. We have had sine sweeps terminated by the shaker due to control issues. Also, you may have some noise floor challenges with the sine sweep since you may have to sweep at an extremely low level to keep the hardware safe. We also allow the shaker to slow the sweep rate through major resonances, 1 octave/minute or slower, and 4 octaves/minutes elsewhere.

    Thank you for all the work you do and all the information that you publish. I can also say that I’m jealous of all the international travelling you have been doing of late.

    Take care,

    Eric Schacher Mechanical Analyst Harris, Space and Informational Systems

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