How to convert g^2/HZ to g ?

[sani63]

How to convert g^2/HZ to g? I have PSD measurements of an operating Hard disk drive. The unit is g^2/HZ. I want to convert this unit to g so I can use equation F=ma to calculate the force that HDD is exerting on the mounting bracket at different frequencies; so then I can use these numbers in promechanica in the load function to capture the vibrational force that the HDD is exerting on the bracket. I would highly appreciate any help.

 

[BrianE22]

At a particular frequency, multiply the g^2/hz by the frequency bandwidth to get g^2. Take the square root of that to get g's. That will either be your peak g's or RMS g's (I don't remember which) at that frequency.

 

[IRstuff]

PSDs are probability functions, so static analysis based on individual frequencies don't necessarily have any meaning.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[rb1957]

isn't a PSD a distribution of g^2/Hz vs Hz ?

so the area under the PSD is in terms of g^2 ??

 

[jimkinney]

rb1957 is correct, take the square root of the area under the PSD curve and you will get an RMS acceleration of the whole spectrum.

If you are only interested in a specific frequency, then you need to use Miles' Equation: Grms = sqrt( pi/2 * f * Q * ASD ). ASD is the PSD value at the specific frequency, and Q is typically (but not always) 10, it's based on the damping ratio if you know that. If you need Gpk, multiply by 3.

Hope this helps.

Jim

Jim Kinney
Kennedy Space Center, FL

 

[zekeman]

Sani 63,

You misunderstand. The PSD is the environmental random vibration. The response to that is, as Jim says, the Mikles equation.
Your hard drive bracket has only a few resonance points of interest . In that case you apply the Miles equation to get the separate responses.

Also, to add to ,Jim's discussion, the Q value is 1/2*sigma where sigma is the critical damping ratio of the single degree of system and 3 factor is the standard 3 sigma probsbility value, but not the peak.

 

[jagad5]

Read the chapter on random vibration in Steinberg's Vibration Analysis of Electronic Equipment. I've read many books on the topic and this one is the best introduction among them.

http://www.bookfinder.com/search/?a...ion%20analysis%20for%20electronic%20equipment

Doug