# Inputs for Frequency response analysis using random PSD data

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In a random frequency response analysis, the input is in PSD in (m/s2)2/Hz and RMS value in m/s2. I need to perform a random vibration analysis in Hypermesh using Optistruct. How we should input PSD and RMS values. What load collectors/cards should we use?.

Next question is regarding units, if Im using mm,Tonne,Newton units. Then how should i convert the PSD data in (m/s2)2/Hz to compatible units.

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Hi,

You can input PSD through RANDPS and RANDT1 where RANDPS hold the PSD specifications and RANDT1 holds the PSD loading Vs frequency data.

RMS is output from random response analysis and not given as input in optistruct.

To convert PSD to mm u have to multiply by gravity

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Hi Prakash

Thanks for the reply. I have given input using RANDPS card only. I couldnt find a load card with RANDT1 in optistruct profile.

Regarding units conversion If i have an input ASD of 0.0298 (m/s^2)^2/Hz. To convert it into compatible units of (mm/s^2)^2/Hz. I should multiply with 9810 mm/s2.

Then it will be equal to  0.0298*9810 = 292.3 (mm/s^2)^2/Hz. Is this correct and what you meant.

My unit system is mm, N, Tonnes

Regards

Meeran

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One of the intelligent guy gave me this !

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On 11/11/2014 at 11:03 AM, Prakash Pagadala said:

You can input PSD through RANDPS and RANDT1 where RANDPS hold the PSD specifications and RANDT1 holds the PSD loading Vs frequency data.

I couldn't find RANDT1 card. I am using HM13.0 & 14.0

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10 minutes ago, Subhu said:

I couldn't find RANDT1 card. I am using HM13.0 & 14.0

Sry. I Have Found. In the Extension of RANDPS card.

Rahul R likes this

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