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Motion Simulation: 6-DOF stewart table

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teookie

teookie

Mechanical
Sep 1, 2010
56
I'm trying to use the NX Motion Simulator for the first time on a 6-DOF stewart table. (looks kinda like this: ) My goal is to create an animation showing the table rise several inches, and if I can, rotate and tilt a little once fully extended.

I first made an assembly with all the various components positioned with assembly constraints. I fixed the base structure that the actuators attach to, and used appropriate constraints on the actuators themselves and their bearings. I did not constrain the actual table, and by using the Move command I could reposition it manually and all the actuators and bearings would adjust to accommodate. So far so good.

I then started Motion Simulation with the NX start button. NX automatically converted my components to links, and my constraints to joints. All appears to be correct and accurate (to me, at least). The base structure is fixed to ground, and I have joints between all the actuators and their bearings, as well as the top of the table. I believe I have all the links/joints set up properly, because I can use Tools>Quick Drag to reposition the table and all the actuator and bearings adjust to accommodate automatically.

Now I need a motion driver to create an animation. Since I just want the table to raise several inches I created a new slider joint on the table top with the direction vector pointing straight up, and set the driver to "constant" and speed to 1 inch/second. I then created a new solution with time = 5 seconds and steps = 500, and making sure the "solve on OK" box is checked. Problem is, after all this when I click play on the animation control toolbar nothing happens.

If anyone knows what I'm doing wrong I'd appreciate some help.
Thanks.
 
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Forgot to mention I'm using NX version 6.0.5.3
 
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Update: I got it to animate, but I had to assign an initial velocity to the table link itself. I have still not been able to animate it via a motion driver which is what I would prefer to do.
 
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I'm still having trouble animating anything using drivers.

I created two simple gears and followed the instructions in this tutorial verbatim, but I still get no animation.


I am hopeful that someone knowledgeable with the software can tell me what I'm doing wrong, alternatively I'll keep trying and post here when I figure it out.

Here is a screen shot of my gears. You can tell that I threw them together quick - the random pattern of holes was just so that I could see them turning. The screen shot shows that the joints and driver are set up correctly.

Untitled.png
 
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anything you can upload ?
Regards,
Tomas
 
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Well , i don't know why it didn't run for you, I opened the assembly, made the "Motion_1" active then pushed the "Solve" button and it spins as expected.
A little curiosity,: Why set the gravity constant to zero ?

Regards,
Tomas
 
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Thank you Toost for taking a look!

Wow. There is definitely something strange going on then as it is clearly not spinning on my machine. I wonder if I have a global setting wrong somewhere? Do I need to get CAD support to come over and reinstall NX? I've run out of ideas...

Anyone else have any ideas what might be preventing my machine from animating the parts? To recap; I CAN animate the parts by assigning an initial velocity to a link, but I CAN NOT animate the parts by assigning an initial velocity to a joint.

Gravity constant shouldn't have any effect on these gears so it's value shouldn't matter. On my other assembly it does though so I'm in the habit of setting it to zero.
 
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Tomas, that worked! The files you sent back animated correctly. I wonder if my default solver settings are incorrect? To test this, I hit solve on your files and then tried to animate again w/o success. My solver broke your files. [sad] (I'm happy to be on the trail of the problem though [smile] )


As another data point, I had a coworker open and solve my original files on his machine and it didn't animate for him either. So I beleive whatever problem is preventing animation is common across all NX installs in my group.

Here are my RecurDyn Analysis settings. Are they the same as yours?

Untitled-3.png
 
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More info. I have found a solution. Sorta.

I changed the solver from RecurDyn to Adams and was able to solve the model and successfully animate. However, I had to move my files to a folder location with no spaces in the path (c:\gears-test\), otherwise I would receive a error stating, "There is some illegal letter & or space in the fullpath filename."

So at least now I can animate my parts like I want to, although it's strange that (1) I can only use the Adams solver and (2) that I have to keep my files in a disk location with no spaces in the path.

Thanks for all the help! I'm going to keep working on this and will post if I learn anything interesting.
 
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Adams has been around ( Like NX) for many years, and is probably cautious on filenames / addresses containing spaces. It is in fact only a few years that NX has supported spaces in file names. Adams probably also has a unix heritage.
- In Unix a file name like "gears test" is two files. ( I stay away from spaces in file names and directories, you never know when you will be use some older or Unix ported code...)

My customer defaults are identical.

I don't know if Recurdyn needs a separate license or not. It could be the reason.

If i was to test if the motion analysis in NX works, i would create a single new part, in that a solid block. Then Start - Motion analysis, create a link of the block, create a revolute joint on one of the edges of the block. ( just pick the edge and nothing more) press the driver tab and set a constant , initial velocity = 360. Ok the dialog and then solve. It should spin 360 degrees in one second.

Btw, how did you make these pictures display in the thread ?

Regards,
Tomas
 
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Tomas, thank you very much for your help.

I tried to make a simple block spin as you suggested, and it didn't work.

What version of NX are you using? I wonder if RecurDyn is broken in my version, 6.0.5.3. I don't think that I'm experiencing a license issue, because it will solve and animate with RecurDyn if I assign the initial velocity directly to a link instead of putting it on a driver. (Problem with this is I am limited to constant acceleration and I cannot control motion with functions.)

I have found that the Adams solver is not ideal for my case since it can't solve for a driver on a cylindrical joint. So I am back to square one. I may have to accept that I am unable to animate the 6-DOF stewart table like I want to.


Now, something I do know how to do. [glasses] To insert an image in a post you put the image URL between image tags. Like this:
 
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Adams should equally be able to solve drivers on cylindrical joints. Adams was the only choice for many many years. Recurdyn has been available for maybe 5 years(?) in NX.
I don't know the exact differences between the solvers but i don't think that it is on this level.
My NX 6 install is a bit f... ( guess the missing word :) It's a 32bit version on Win7 64 , NX 6.0.x ( the initial release), i downloaded the 6.0.5 update the other day, now tried to update only to see that i downloaded the 64 bit version. But , it does animate.


Regards,
Tomas


 
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