Problem with flexibility

[Anonymous]

Hello, 

I have a assembly of pneumatic cylinder. In master positional representation all parts are fully constrained. This pos. rep. I use for drawing. Also I have positional representation named "movable". In this "movable" rep. is suppressed one constrain so piston can move in cylinder. Pneumuatic cylinder is placed to he assembly and it is set to flexible in the browse bar. Also the positional rep. of pneum. cylinder is set to the "movable" (because standard master is fully constrained). It generally works but sometimes cylinder breaks up. It means that it ignores the constraints in the pneumatic cylinder and some parts move against each other although they are fully constrained against each other. My expectatiton was that cylinder will move only according to degrees of freedom.

Does someone have some experience with it? Am I doing something wrong or Inventor flexibility has some bugs?

JT 

[mdavis22569]

Can you share it ... I've done this before ..and so have quite a few others on this forum. 

 You might be over constraining, or have conflicting with Pos Reps/ constraining/flexible

[jtylerbc]

Is the body (or some other portion of your cylinder) grounded?

Grounding one part and constraining the others to it is a common general assembly modeling technique, but can cause some problems when it comes to Flexible subassemblies in particular.  Instead of grounding your cylinder body, constrain it to the assembly origin and see if that improves the behavior. 

For whatever reason, grounded components in flexible subassemblies don't always behave the way they are supposed to.  At my company, we struggled intermittently with this exact issue (except on hydraulic cylinders) for quite a while before discovering that the bulk of our problems were being caused by this grounding issue.

[johnsonshiue]

Hi! Is it possible for you to post an example here or send it to me directly (johnson.shiue@autodesk.com)? We have a regression in 2017 that a subassembly is placed with non-Master PosRep while top-level assembly is in Master PosRep. I would like to take a look at your case and see if it is a known issue.

Many thanks!

[Anonymous]

Hello,

I have tried your advice and I´m confuced. Sometimes it works, sometimes doesn’t. But my flexible assembly is maybe a little bit complicated because there are more flexible parts which are constrained to each other and some of them has also another flexibility inside itself. So this is multiple flexibility and maybe this is the problem. And maybe for one level flexibility your procedure is functional.

[jtylerbc]

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@Anonymous wrote:

But my flexible assembly is maybe a little bit complicated because there are more flexible parts which are constrained to each other and some of them has also another flexibility inside itself.

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I haven't seen your assembly, so I could be interpreting what you're saying incorrectly. I think what you are describing is that you have a nested assembly structure with flexible assemblies inside other flexible assemblies.  If that is true, you will need to constrain to the origin instead of grounding at every level that is used as a flexible subassembly.

[johnsonshiue]

Hi! Could you send me an example directly (johnson.shiue@autodesk.com)? I would like to understand the behavior better.

Many thanks!

[chad.schmidt]

I'm also seeing the same issues.  Exploding flexible assemblies using position reps.

[johnsonshiue]

Hi Chad,

The behavior is usually files specific. Could you share the files with us? Please send them to me directly (johnson.shiue@autodesk.com) or share it with me on a cloud drive of your choice (A360 is free).

Many thanks!

[jtylerbc]

In addition to sending the files to @johnsonshiue for troubleshooting, did you already try the fix I suggested earlier in the thread? 

[chad.schmidt]

I've tried a little bit of everything with no success.  I've sent the files to Johnson, hopefully he can spot the problem.

[johnsonshiue]

Hi Chad,

I took a look at the files you sent me. This is the same as the following thread. It is not related to flexibility. It is related to creating a new top-level Positional Representation when one of the subassembly is in non-Master Representation.

https://forums.autodesk.com/t5/inventor-forum/2016-positional-rep-display-problem/td-p/6947962/highl...

As I mentioned earlier, we are working on a fix and hopefully it will be available soon.

Many thanks!

[Anonymous]

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@jtylerbc wrote:

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@Anonymous wrote:

But my flexible assembly is maybe a little bit complicated because there are more flexible parts which are constrained to each other and some of them has also another flexibility inside itself.

---

I haven't seen your assembly, so I could be interpreting what you're saying incorrectly. I think what you are describing is that you have a nested assembly structure with flexible assemblies inside other flexible assemblies.  If that is true, you will need to constrain to the origin instead of grounding at every level that is used as a flexible subassembly.

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I tried it with all levels od subassemblies constrained to the origin instead of grounding and it is the same (only master assembly had grounded component - probably I will try it again with master assembly without grounded component). It behave sometimes bad sometimes good. But what I tried another thing and it mostly worked. I grounded problematic components in the subassembly (components which you don´t want to move). So the components were put in the correct position by constrains and then were grounded. But it is not good way to create 3D models. It is just the opposite experience than yours.

Regarding grounding I thougt that Inventor makes some constrains to the origin on the background but according to different behaviour in the case of constrained/grounded component it is not probably true.

Regarding the data - our company policy doesn´t allow to share the data.

[jtylerbc]

@johnsonshiue seems to think it is a different issue (one I'm not really familiar with), so it could simply be a case of two problems that can cause similar symptoms.  What you are seeing is what it often looks like when you have the "grounding" problem I described, but that may not be the only thing that can cause it to behave that way.

[johnsonshiue]

Hi Jan,

Based on the description, Chad's issue is different than yours. Chad's issue is a know bug in 2016 and 2017. The scene does not properly update when a new PosRep is created at the top level. We are working on a solution for that.

Your issue is more complicated than that. Based on my past experience, there are several possible causes: one or more constraints misbehave; an angular constraint flips direction; DOF in the flexible sub is ambiguous; geometry has tolerance issue leading to unstable solve results and so on. There are just too many variables attributing to the behavior you are seeing. It is better if you can share the data with us. Otherwise, there isn't much we can do.

Do you have access to Autodesk Product Support? If yes, I encourage you to contact Autodesk Product Support. I can also ask the team to contact you if you want. We need to take a look at the files to understand the behavior better.

Many thanks!