Sherlock Damping Factors

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Nathan Blattau
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Sherlock Damping Factors

Post by Nathan Blattau » 28 Mar 2014, 11:31

Sherlock uses mode based damping computed from the natural frequencies. The damping factor field supplied in the analysis properties (random, harmonic) is a modifier for the computed mode based damping factors.

If you want to look at the computed damping factors for comparison factors you need to look at the generated .inp file for the analysis located in the project folder. For example:
C:\Users\nblattau\AppData\Roaming\Sherlock\projects\ODB++ Tutorial\ODB++ Tutorial\modules\HarmonicVibe\calculix-HV.inp

Open the file with notepad (or a better text editor), scroll to the end and you will see the computed damping factors:

** End of Model
**-------------------------------------------------------
**-------------------------------------------------------
** HARMONIC VIBRATION ANALYSIS STEP
**-------------------------------------------------------
*Step
*Steady State Dynamics
1.0E01,1.0E03,5,3
**
*MODAL DAMPING
1,1,7.3452E-03
2,2,1.4526E-02
3,3,2.1379E-02
4,4,2.5118E-02
5,5,3.1541E-02
6,6,3.6158E-02
7,7,4.0499E-02
8,8,4.3865E-02
9,9,4.7411E-02
**
*Amplitude, name=HarmonicAmpl
1.0E01, 1.0E00
1.0E03, 1.0E00
**
*DLOAD, AMPLITUDE=HarmonicAmpl, OP=NEW
ALL_ELEMENTS,GRAV,9.8066E03,0.0E00,0.0E00,-1.0E00
**
*node file, frequency=1, output=3D
PU
*el file
E
*endstep

You can increase or decrease the damping with the Sherlock damping factor property. Numbers below one reduce the damping (a straight multiplier), 0.1 multiplies all the computed damping factors by 0.1.
Nathan Blattau, PhD
Senior Vice President
DfR Solutions, LLC
9000 Virginia Manor Rd, Ste 290
Beltsville MD 20705

hakwon
Sherlock User
Posts: 8
Joined: 13 Jun 2013, 19:00

Re: Sherlock Damping Factors

Post by hakwon » 07 Jul 2015, 22:16

Dear Sir,

If the damping factor is computed based on the Natural Frequency of the board, the preset damping value at the time of vibration simulation seems to be the computed one.
If I need to define a modifier by setting a different valaue on the damping field from 0-1, what would be general guidelines on the specific numbers. For instance, o.1 for rigid mounting up to 1 for chassis mounting and then the intermediate values?

Best Regards,
HA

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Nathan Blattau
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Posts: 121
Joined: 21 Mar 2013, 16:40
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Re: Sherlock Damping Factors

Post by Nathan Blattau » 14 Jul 2015, 13:20

The damping factors are based on the natural frequencies of the pcb. During random or harmonic simulations, Sherlock does a natural frequency simulation first and then computes the damping factor for each mode.

The analysis property in Sherlock is used to scale these already computed factors. I use 0.2 to 0.3 for very rigid mounting conditions (decreases damping) and a value of 1 for a pcb mounted in a chassis.

Best regards,
Nathan
Nathan Blattau, PhD
Senior Vice President
DfR Solutions, LLC
9000 Virginia Manor Rd, Ste 290
Beltsville MD 20705

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