Issues with designing a fixture for ESS Vibration Test

[AhmadA]

Greetings everyone,

I am designing a fixture for a camera to be tested on a 2-axis vibration machine as per ESS standard, which means it will undergo random vibration ranging from 5 to 2000 Hz. The client says that the fixture's resonance should be way far from the testing range (Almost 4000 Hz). I have tried over 50 conceptual design until this moment but I failed to deliver because the camera is mounted from the side.

I have 2 questions that'll help me a lot:

1- Does the tested equipment (EUT) have to be tested in the exact same orientation as the actual application or is it not that important in ESS vibration tests? Because I have conceptual designs that have very high natural frequencies but the drawback is that the EUT has to be flipped 90 degrees in order to be mounted.

2- Is it possible to design a fixture that has natural frequencies within the testing range but will be suppressed by dampers and rubber mats? Or will it affect and interfere with the desired results?



I hope to find some answers because this has been giving me a hard time!

Regards,

Ahmad

 

[GregLocock]

4000 Hz sounds like a ridiculous target. How much does the EUT weigh?

1 that depends on the system. For instance if you were testing a car on a fatigue rig you'd normally have it with gravity pointing downwards. But if you are applying 1000g shock pulses it wouldn't matter.

2 Doesn't sound likely.



Cheers

Greg Locock


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

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

 

[IRstuff]

1> Depends, but in general, not, since the purpose of ESS is to weed out workmanship and infant mortality failures. That said, however, if your current configuration is in the normal usage orientation, I'd have to wonder if this isn't going to cause long-term problems, e.g., fatigue on the mount, and/or amplification of actual usage environments that could shorten life.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[AhmadA]

Hi GregLocock and IRstuff,

The reason it is 4000 Hz is to limit transmissibility (output/input). If the resonance is within the range transmissibility would reach a very high number (Infinity theoretically).

Anyhow, the weight of the camera is 20 Kg, and it will be mounted from the side at one end only and hanging free from the other end. This is why changing orientation would be very beneficial, because it will allow me to flip it and mount it directly to the vibration table.

Regards,

Ahmad

 

[GregLocock]

"The reason it is 4000 Hz is to limit transmissibility (output/input). If the resonance is within the range transmissibility would reach a very high number (Infinity theoretically)." Thanks, I may have picked that up at some point in the last 40 years working with vibration.

So now we've got a number 4000/2/pi=sqrt(k/20)
k=8 kN/mm

That's approximately as stiff as a typical steel suspension member on a car. But they resonate at frequencies in the 400-2000 Hz area. So you need something that is something like 10 times stiffer per unit volume than steel, or massive chunks of steel.

As to your other point it depends on the input load spectrum and what you are simulating. I would GUESS that EUT orientation isn't that important.


Cheers

Greg Locock


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

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

 

[IRstuff]

What's easy for you might be nonsensical for the EUT. If you are restricting the ESS to only 2 axes, then they probably should be both in plane with PCBs within the EUT.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[GregLocock]

Presumably the input load spectrum has some directionality. Since your shaker only works in one axis(?) does that not fix the orientation?

Cheers

Greg Locock


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

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