- In this video, we're going to look at
transient dynamic analysis but using in force base motion.
We're going to use a modal method.
We'll start off by using just a single mode and
then we'll repeat the exercise using six modes.
We'll open up the Solidworks file
which is called tray3_begin.
And again if you don't have Solidworks 2018 or later,
then you use the corresponding file
you'll find in the exercise folders.
After the file is open, we go to the study tray-lumped mass.
This was a normal mode analysis.
We can see the 10 modes that were calculated
down in the feature tree.
So we can review these very quickly.
Here's mode one, mode two, mode three, mode four,
mode five, mode six, mode seven, mode eight,
mode nine, and mode ten.
Now, if we review the mass participation table,
we'll see what modes are likely
to play a key role in the base motion.
Now, mode one has 86% of the contribution.
The other modes, mode two, mode six, play a small role.
The total contribution is 93%,
but it's pretty clear out of that,
that mode one is really going to be the dominating mode.
Let's create a new study to carry out the base motion.
Use your space to copy the current one, so copy study.
We choose a linear-dynamic analysis option,
and then choose a transient,
and then we can name this study base_acceleration.
Now we right mouse click on the study name,
set up the properties.
For the first exercise, we just run with one mode.
I will set up a duration of .031 seconds,
a time step of .0005 seconds,
the calculations behind these values
are shown in the supporting videos.
Let's just show the mesh.
Right mouse click on external loads,
and we're going to set up uniform basic citation,
going to choose acceleration input,
and we choose the direction along the edge.
The values we want in inches per second squared,
I want to use 10g,
which is 386.4 times 10 inches per second squared.
Now we're going to define a curve.
Now we're going to define a half sine pulse
and use harmonic loading for that.
Now be careful to set the units into hertz,
and have a start time of zero,
and an end time of .01, or 10 milliseconds.
Amplitude is going to be unity,
the frequency will match at 50 hertz.
Details of this calculation
are shown in the accompanying videos.
Now we can view the curve,
and clearly we can see the half sine pulse.
Now the basic citation is complete,
so we just close out the form.
Now right mouse click to set up the damping.
Now we only have one mode,
so we're going to set that to 2% critical damping,
check when done.
Now let's just double-check the analysis duration
and the time step.
So now we can run the transient, so click run this study,
again I've accelerated the timeline
so that we can go straight to the results.
The result plots by default as showing the 10 mode shapes.
We want to go in and find the results for the transient.
Right mouse click on results and choose displacement plot,
and we want to have zed displacement.
We want the fringe to be discreet,
and the boundary to be mesh.
Then just check okay.
Now we see the zed displacement time step 62,
which is .031 seconds.
Now let's just tidy up the results
by getting rid of the mode shape plots.
Now let's produce a response graph
of the center of the payload mass.
Right mouse click on results, choose define response graph,
and then we're going to select the node at the payload,
going to choose acceleration, zed acceleration,
change to inches per second squared, and then check okay.
Now we see the acceleration time history.
If we expand it, we can see the time under the impulse
and then the time under free motion.
We can also see some slight clipping
because we've kept only 10 time steps per cycle.
We'll see in the next little exercise
that we can actually increase that number of time steps
to improve the fidelity.
Let's modify this response graph, so edit the definition.
What I want to do is change the units of acceleration to g,
and if we replot, we can simply see it's now calibrated in g
rather than fundamental units of inches per second squared.
Now let's animate the displacement response,
so right mouse click on displacements, choose animate.
Let's set the framerate to three,
now you can see the response.
It's a little bit choppy
because we don't have very many frames during this signal.
We can never see the general trend of the response.
Now I want to re-run the transient analysis
but allow six modes to be used in the calculation.
We're going to see if the high-frequency content
shows up in our response graph.
Now the first task is to change the settings of our analysis
so right mouse click analysis, change the properties,
and I want to allow all 10 modes,
potentially, to come through,
so I change the number of frequencies to 10.
Under dynamic options,
I have a smaller time step requirement,
so change that to .00011
and that caters for up to mode number six.
Again, the calculations are in the accompanying video.
Now I have to adjust the damping
so it points to all of the six modes
that I'm going to be using,
so the last mode now becomes mode number six,
and retain the .002, the 2% critical damping.
Now we need to rebuild the modal database
to have all the modes available that we need to pull from.
To do this, I need to re-run the normal modes analysis,
here I hit frequency.
I sped up the timeline so we go straight to the results,
and now I have the results of 10 natural frequencies
on normal mode shapes available to me.
Now we go into result options
to choose those modes that I want to use.
So, selected modes, we edit,
and then all 10 are available,
and I'm just going to run with the first six.
So this is rebuilt, the modal database,
which I'm selecting from.
Check okay, and now we're ready to run.
So select run this study.
Again I've sped up the timeline
so we can go straight to the results.
We open up the results
and we're going to go to the response graph, show that.
We can see the new response.
If I look at this, I can see that
we've got very high fidelity,
whereas before we had clipping.
Now there's no clipping,
but what's very clear is that
we're only getting rid of the contributions from mode one.
So although we included possible contributions
from mode one through six,
we haven't seen any additional mode shapes coming through.
There's no high frequency content.
The smaller time step was allowed
higher fidelity and a smoother curve.
And now we'll save the file,
and we'll save it as tray3_end.