Several great suggestions were mentioned (summarized below) but this simple
problem is wupping us. Neither the Stiff Spring method of Enforced
Displacement or the Base Excitation method are returning expected results.
Either we are expecting the wrong thing, doing it all wrong, or Mechanica is
broke and needs fixing.
See this webpage for a description of the problem with screen shots.
http://www.solidpe.com/BaseExcitation.htm
First, using the Stiff Spring method recommended by Mr. Holst we were
getting enforced displacements of 0.0025 inches with 125 lbf acting at a
1000 lb/in spring...way short of the 1/8" that that force should have
produced. Besides, the harmonic (16.xx Hz) of the ruler itself was showing
up in the (spring mounted) base even though the spring is way stiffer than
the ruler.
Second, the Base excitation is returning screwy Base Displacements (see
webpage above).
As Bill O'Reilly says, "Where am I going wrong...?"
SUMMARY:
=== Yuri ===
1. It looks like you should run Frequency response -- range from 10Hz to
20Hz, just make sure you have 15Hz (0.5 interval between 10 and 20Hz)
requested in Output tab. Then you'll have 2 rabbits shot with 1 bullet (as a
Russian saying goes...) -- get full results at 15Hz, as well as get the
system's response from 10 to 20Hz.
2. Base excitation in Pro/M is defined as acceleration, so you should
calculate the acceleration amplitude of the 'base' (pretty trivial task
since you know the displacement amplitude and frequency) in whatever units
you use for your model (e.g. m/s2, or in/s2, etc) -- given your desciption
of the system, it looks like you should end up with a 'parabolic'
acceleration amplitude function (i.e. acceleration proportional to frequency
squared) -- then enter this function in the analysis definition form.
3. Since you'll be running base excitation, use mass participation factors
to ensure the accuracy of your analysis. The total should be 0.8 or greater
(0.9 or greater, I believe, is required by US Navy in their shock analysis
requirements...), otherwise you'll need to use more modes (determined in the
Modal analysis used for the Dynamics).
=== Jim ===
I don't know if it helps or if useful, but I show a method for doing
Enforced Dynamic Displacements in the Tips&Tricks section of my web site.
www.TSDengineering.com
=== David ===
1. Base Excitation is precisely putting your acceleration inputs at the
constraints. And as we all know Force and acceleration are related when
mass is involved. Think of it like being an earthquake. Normally you
constrain the base and it's rock solid, but then that solid base itself gets
wiggled. Your option is either to do base excitation (vibrating the
constraints) or to oscillate an applied load. So you can'y apply an
enforced displacement and oscillate that (Not to my knowledge at least).
=== Rami ===
I'm not entirely sure that you need to use vibration analysis in mechanica.
According to your description, at least as I understood it, you better use
MDX (mechanism analysis). The beauty of it is that after you have calculated
what you need/want in MDX you can transfer it directly into mechanica
(structural).
The vibration analysis in mechanica is great for detecting small deflection
inside the model or an assembly, however I found that MDX is better & easier
for the rest. Now since you know your input I don't think you will have any
difficulties.
END OF SUMMARY
Regards,
Gavin
.png "GavinBRumble"){kind=avatar}
