Static vs Dynamic Forces - Trouble Interpreting Vibrations in Data

[toothless48]

Hello all,

I am working on an analysis of a large chassis frame for an autonomous vehicle. We are validating the performance capabilities of the system using vehicle-mounted accelerometers. The raw data that we are collecting is filled with intermittent high-amplitude noise, most likely due to vibrations when the vehicle drives over rough patches in the concrete floor. This system follows a motion command array, but the system's true performance capabilities are not currently known.

The intention is to take the collected data, and feed it into static load cases for an FEA study. The current plan is to filter the data to remove "non-commanded" motion, and then use the peak filtered results for the static analysis. My question is this: how do I interpret the high-frequency vibrations in terms of structural analysis? In another way, at what frequency can dynamic loading be modeled as static loading? I am of course not looking for a numerical or concrete answer, but maybe for some guidance from anyone who had similar experience. It seems fair to say that 100Hz frequency components on a structure similar to a truck are not static. The vehicle design and hardware are vibration-tolerant, so I am really just looking at this from a structural standpoint.

Thanks very much

 

[drawoh]

.... The raw data that we are collecting is filled with intermittent high-amplitude noise, most likely due to...

Until such time you determine that the high frequency vibrations are spurious, I would treat them as part of my data that must be accounted for in my static analysis.

--
JHG

 

[rob768]

High frequency vibrations usually relate to small displacements. stresses will be low.

 

[GregLocock]

Except when they excite resonances...

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[rob768]

Was indeed thinking of mentioning that. This remark is especially valid if the frame resonances excite parts like oil pumps and general small machinery attached to the frame. But, 100 Hz in a large frame usually relates to local resonances which can easily be tackled by adding some local stiffeners.

 

[drawoh]

rob768,

...intermittent high-amplitude noise...

--
JHG

 

[GregLocock]

Yeah but high amplitude acceleration != high amplitude displacement, at high frequencies, and strain is proportional to displacement.

To be honest I think the OP is on a hiding to nothing, using accelerometers to generate vehicle level durability loads has not, so far as I know, reliably produced results that are sufficiently useful. This is a cause of great angst in a program management sense, since a durability load car probably takes several weeks to instrument up, and then the load cells and strain gages randomly fail during the tests, and then somebody recalibrates some channels during the test and plugs the wrong lead into the wrong place, and so on and so forth. Once you have the test data, ie loads in various places, some lucky person has to turn them into a coherent set of loads to drive the FEA model, a non trivial task in itself- figure on up to 1000 times slower processing speed than real life. By the time all the fun and games is over it can take several months to provide a set of loads to the FEA guys that are reasonably useful and believable. Which means they were tested on a way out of date vehicle.

It would be much easier to just to slap a bunch of triax accelerometers all over the car and call the job done, but like I said, it doesn't seem to work.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[FeX32]

Sensing strain is much more reliable.