Sine Sweep Anomaly?
Sine Sweep Anomaly?
(OP)
Hi,
It's not my area of expertise, but I was asked if I had any comments on a 500 Hz resonance that appeared on the .5G sine sweep (20Hz-2000Hz) of four different electronics chassis. I didn't perform the FEA models for these units, but the analyses didn’t predict these resonances. In one case the analysis predicted no resonances in the 20Hz to 2000Hz range. The common natural frequencies between the four units seems faulty (they have different geometries and masses). Any suggestions on what to have the techs look for in regards to this anomaly?
Thanks.
It's not my area of expertise, but I was asked if I had any comments on a 500 Hz resonance that appeared on the .5G sine sweep (20Hz-2000Hz) of four different electronics chassis. I didn't perform the FEA models for these units, but the analyses didn’t predict these resonances. In one case the analysis predicted no resonances in the 20Hz to 2000Hz range. The common natural frequencies between the four units seems faulty (they have different geometries and masses). Any suggestions on what to have the techs look for in regards to this anomaly?
Thanks.





RE: Sine Sweep Anomaly?
Objects that are top-heavy can sometime affect typical floating slip tables, causing the table to rock on the oil film, creating a false resonance.
Our system sounds to be much heavier, e.g., about 800 pounds and 5 ft tall. A hydrostatic bearing table doesn't have this sort of problem.
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Sine Sweep Anomaly?
Box (10" x 8") ~ 18 lbs
Box (10" x 9") w/2 rectangular modules that are side by side with about a 1.5 inch gap in between. ~21lbs
Box (10" x 9") w/2 rectangular modules that are in corners diagonal from each other. ~21 lbs
Box (13" x 10") w/ 8 slices on top sandwiched together and supported by long brackets on the top and sides. It is a little shorter than the units with just two modules on top. ~27lbs.
It would be interesting to note that this resonance will show up in each axis. Actually, its about 600Hz +/-50 but there are some that are closer to 500Hz. It just seems strange. I guess due to the spread it is possible, but it seems to indicate some resonance in the system. Could a faulty accelerometer have a resonance that contributes to the measured acceleration?
<<Is there anything unusual about these chassis'? >>
Not really. Two of them are very similar. I wouldn't be bothered if they showed very similar resonances, but the light unit and the heavy unit share the resonances as well.
RE: Sine Sweep Anomaly?
Tobalcane
"If you avoid failure, you also avoid success."
RE: Sine Sweep Anomaly?
If the latter, then oil canning sounds more plausible.
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Sine Sweep Anomaly?
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Sine Sweep Anomaly?
GregLocock: For between 20Hz and 200Hz we see the .5 G acceleration that is supposed to be the input. Would I be looking for a force input change at that resonance?
Thanks everyone for their help.
RE: Sine Sweep Anomaly?
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Sine Sweep Anomaly?
I have a couple questions:
(1) Does a dip in the force psd correspond to a unit resonance?
(2) A force peak above the base input indicates a shaker issue. The issue could be a number of things right? It could be something in the shaker resonating or a problem with our force input system?
Best Regards.
RE: Sine Sweep Anomaly?
(2) yes, obviously it depends on the exact layout of your shaker system. You need to measure the force where it is applied to the table, not just the inferred force from the drive signal.
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Sine Sweep Anomaly?
RE: Sine Sweep Anomaly?
Is your base accel a 3-axis accel or only a single axis?
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Sine Sweep Anomaly?
IRstuff: The base accel is 1 axis at a time. This resonance appears in each axis. When one axis is excited it is apparent in the cross talk measurements of the other two axis.
RE: Sine Sweep Anomaly?
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Sine Sweep Anomaly?
RE: Sine Sweep Anomaly?
to eliminate any questions about the slip table itself, you'd need a vertical accel close to the end of the table to measure any rocking motion.
Are the accels that were showing resonance attached to the face of a large area? Were there any accels attached to a presumably stiffer structure, like an edge? Were there differences in response?
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Sine Sweep Anomaly?
The accels were attached to a variety of surfaces on the units: Small panels and Large panels. The accel responses were very uniform in each axis. The unit as a hole is stiff.
RE: Sine Sweep Anomaly?
Sounds like a table problem, then.
TTFN
FAQ731-376: Eng-Tips.com Forum Policies