Flexibility in Assemblies - still a thing of the impossible

[JoschaWondraschek]

Hi folks,

I'm just curious on how many of you use flexibility in assemblies and how you use it.

I mean, it's 2020 and Inventor still can't handle/calculate flexible assemblies correctly.

If you try to work with 2 or more flexible assemblies, it's like:

Parts disappear. Everything turns 180°. You get an unwanted explosion view. In the distance sirens.

Inventor acts like Microsoft Word of year 2000, when you moved one image 😅

It's really frustrating, cause it would ease up work, if they get this working correctly.

Josh

[mcgyvr]

I use flexibility in every top level assembly we have without issue (250+ assemblies) and the flexible subs are on average 50+ parts.. 

There are a few users.. @Cris-Ideas  (<--I think it was Cris)  who have reported flexibility issues in the past and there has been some agreement that it could be improved a bit I believe.. 

Do you have a repeatable example/dataset you can post?

[jtylerbc]

I do it pretty regularly as well. I have certainly ran into some problems at times, but most of them eventually turned out to be self-inflicted.

They can be a bit finicky, and there probably are some genuine bugs and instability involved in some cases.  But most of the time when I've had trouble, it turned to be some kind of problem with the way it was set up in that model, rather than an issue with Flexibility as a whole.

[alewer]

I have little experience with "vanilla" flexibility, but I have a long and painful history where I need both positional reps and flexibility. When I say long and painful, I mean beginning with maybe Version 11 (not 2011) with multiple cases through resellers, Autodesk support, forum posts, etc. My assemblies would "solve" with multiple constraints violated but Inventor reported zero errors. The behavior that you describe where components positions and orientations go askew matches my experience. I had not however noticed the distant siren behavior that you describe. Here is one example.

The good news is that with my limited testing of Inventor 2021 there is a real improvement with my models and workflow. In my limited testing time, I have not been able to recreate the buggy behavior in 2021. As far as my workflow goes, the flexibility issues might be fixed as of version 2021.

One final point: it is really important to make sure that you are doing everything possible to avoid assembly constraints that allow for multiple solutions. If you are seeing parts rotate 180°, it might be that this is a solution in that constraints are solved in a way that is undesired but mathematically valid. To avoid assemblies with multiple solutions to constraints I try to use plane to plane mate and flush constraints, and the directional constraint options (added 2020?) are also a big help here in making sure that there is no 180° solution. Unless angle constraints are 0° or 180° I get instability (multiple solutions), even with the directed angles.

[johnsonshiue]

Hi Josh,

Indeed, we have issues in Flexible assembly and PosReps workflows. We attempted to resolve those issues with half success. Sometimes it introduced more issues than before. It is problematic.

Like Aaron mentioned here, we did make tremendous improvement in this area on Inventor 2021. All the known issues in this area have been resolved. But, I am sure there will still be unresolved bugs. If you can share the files with me (johnson.shiue@autodesk.com), I can help take a look and see if it works better on 2021. I am sorry that the improvement on 2021 cannot be ported back to 2020 unfortunately.

Many thanks!

[JoschaWondraschek]

First of all thanks for all your input and experiences.

I want to share my experiences so far, maybe I can share a screencast with you.

• It seems, that flexibility doesn't like flexible components in sub assemblies.
• Sometimes even grounded components start to move, if you have flexibility in sub assemblies
• I often experienced, that the flexibility worked when i created a new PosRep, but when I switched to another PosRep and back, it got faulty and didn't work. That's why I think it's a problem with Inventor itself resolving the flexibility.

You can see in the screencast, that the flexibility works, when the same PosRep in the sub assy is activated, but doesn't when you activate a different PosRep in the sub assy. That's some buggy sh** 😵

@mcgyvr: Since it's it's a project we're doing and due to confidentiality agreements I can't post the assy here in public.

@jtylerbc: I looked at my constraints really closely but couldn't find any faulty constraints. I even tried different ways to constrain the assembly. I always try to use planes, axes and points for constraints.

@alewer: Thanks for this hint. Will try to avoid multible solutions for the movement.

@johnsonshiue: If it's not going public I could share the buggy assembly with you, since it seems you're working for Autodesk.

Josh

[jtylerbc]

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@JoschaWondraschek wrote:

• Sometimes even grounded components start to move, if you have flexibility in sub assemblies

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I would suggest that you avoid using grounded components anywhere in assemblies using Flexibility.  Constrain to origin planes instead.

I've had plenty of times that an assembly that was behaving badly would instantly become more stable as soon as I went through it and eliminated anything that was grounded.

[alewer]

It sounds as if you are indeed using positional reps + flexibility overrides. I want to be clear that avoiding multiple solutions to constraints is (in my experience) can help a little but is not going to solve the problem. I wrote an addin that does help with the issue in pre-2021 releases of Inventor. I created a video that shows the addin here.

Video notes:

0:08 Open assembly. Note that my addin is not runninig at this point.

0:17 Activate positional rep with flexibility. Multiple assembly constraints solve incorrectly but no errors reported. No change with click and drag.

0:27 Demonstrating a broken constraint (Flush:2). Inventor is reporting no errors and seems to believe that this and other broken constraints are healthy.

0:47 Enable the addin.

0:52 Reopen same assembly.

0:54 Activate positional rep with flexibility. The assembly solves almost correctly. You might miss it, but the cylinder rod end position solves incorrectly (this violates a constraint).

0:55 Click and drag on components shows correct behavior. Assembly constraints solve correctly after click and drag.

The addin is not perfect but for me and my coworkers it has resolved many of our flexibility with positional reps problems. If anyone is interested in this addin or dataset from the video please let me know and I will figure out a way to share it with you. To be clear I am talking about giving the addin away for free.

[JoschaWondraschek]

@jtylerbc: Sorry for the late reply, but I've been on vacation the last weeks...

Yeah I'm using this workaround (constraints to origin) already for a long time,

but I think you agree with me that "grounded" should mean "grounded" und not "maybe grounded" 😉

[JoschaWondraschek]

@alewer: nice addin! Thank you for your offer.

I would love to try this. PM me and we will figure out how we share the addin.

[JoschaWondraschek]

I noticed another weird behaviour.

Lets say you have a main assembly with PosReps and a sub assembly (also with PosReps and flexibility).

If the main and sub assembly are open simultaniously and an different PosRep is activate in the sub assembly than in the main assembly, the the main assembly has problems resolving the PosReps.

If you close the open sub assembly, the resolving in the main assembly works better, but not perfekt.

Does anyone notice a similar situation?

(I hope I could describe it understandable enough, as english isn't my motherlanguage 😅)

[jtylerbc]

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@JoschaWondraschek wrote:

Yeah I'm using this workaround (constraints to origin) already for a long time,

but I think you agree with me that "grounded" should mean "grounded" und not "maybe grounded" 😉

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I don't disagree that it should work that way - I was just pointing out that it currently doesn't.  For whatever reason, grounding a component in a Flexible application tends to introduce instability.  It usually works fine at first, then freaks out later.