Learning Forum

[deepesh.p.gurdasani]

What is the difference between keeping 'Absolute' to On or Off in Random vibration, will there be change in results ?

Thanks

[peteroznewman]

I explained it clearly in this discussion.

/forum/forums/topic/base-excitation-absolute-result/

[deepesh.p.gurdasani]

Can I say it in this way: 

When 'Absolute' settings is set to YES, the base will move and a part of deformation is taken by the base and the rest transferred to the rest of the structure?

AND

When the settings is set to NO, the base doesnt undergo any deformation and ALL of the excitation is transferred to the rest of the structure ?

[deepesh.p.gurdasani]

Please correct me if I am wrong.

[peteroznewman]

I would say it more definitely.  When Absolute is set to YES, zero on the deformation ruler is on the ground while when Absolute is set to NO, zero on the ruler is on the moving base. It's all about whether zero on the deformation ruler is fixed in space or moves with the base.

[deepesh.p.gurdasani]

if deformation ruler moves with the base when Absolute is set to NO, why are we seeing that 11 mm deformation at point B ? Why is the deformation transferring from from fixed point A to point B ? 

May be I am missing out on some understanding here.  

[peteroznewman]

Imagine a large mass hanging from a long, low stiffness spring. Say the spring is 300 mm long before the mass is attached, and the spring stretches to 600 mm long when the mass is hanging from it.  You hold the top of the spring (point A) in your hand, so your hand is the base in a Transient Structural model. You move your hand rapidly down 11 mm then up to the initital position and hold still. The mass at the other end of the spring (point B) hardly moves at all. That description is from the point of view of an observer standing on the ground (Absolute=Yes).

Now imagine an insect sitting on your hand, what does it see?  It sees your hand has not moved at all, but it sees the mass at the other end of the spring rapidly move 11 mm closer and return to the initial position (Absolute=No).

[deepesh.p.gurdasani]

Also, will this setting have any effect on response PSD or any output results apart from deformation?