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Vibration from Car Engine During Morning Commute

I was curious to see the type of vibration that an engine exhibits when the car is running. So I put a Slam Stick (now enDAQ) shock and vibration sensor on the engine of my "sexy" Saab one morning when I drove to work!

What is a good sampling rate on this application?

For this particular test I sampled the accelerometers at 10,000 Hz. But depending on what you're looking for that may be higher than needed. If you are just interested in the dominant frequencies of an engine's vibration you can probably sample at around 2,000 Hz. A typical car engine's crankshaft is going to rotate at several thousand revolutions per minute (RPM). Even a RPM of 6000 only corresponds to a vibration frequency of 100 Hz. Now there may be some two-per-rev vibrations but you'll still only be interested in vibrations of several hundred Hertz at most.

But if you look at the PSD of an area in the test, you'll notice some pretty healthy broadband vibrations above 1,000 Hz. And this vibration may be more indicative of some subsystem in the engine and its structural health. And if you are interested in more structural health information for fatigue or failure diagnostics, a higher sample rate will provide more information about the environment that you'd miss with a lower sample rate.

So, as is the case in my vibration testing applications, the answer depends more on what you're looking for than it does on the environment or application itself.



The spectrogram is especially cool because you can see different periods of my drive that I'm on the highway and/or accelerating and revving the engine.

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