topic Re: Joint motion problem in Fusion Design, Validate & Document Forum

Joint motion problem

Anonymous — Fri, 15 Jan 2016 22:12:27 GMT

Here's a problem that's been vexing me for a couple of days. As shown in the image below, there are three revolute joints and one slider joint. The slider joint is on the pushrod that slides in an out of the hydraulic pistion. When all the joints are active the whole system locks up and nothing moves. If either of the top revolute joints are supressed, all the other joints move as they should and they move in combination. That is to say, dragging any one part causes all the linked parts to move properly. As soon as I unsupress the joint, the whole thing freezes.

Re: Joint motion problem

TrippyLighting — Fri, 15 Jan 2016 22:37:47 GMT

This is hard to evaluate without acsess to the entire model. Could you share the model ?

Re: Joint motion problem

Anonymous — Sat, 16 Jan 2016 01:28:28 GMT

Yes, I'd be happy to share the model. How, exactly do I do that.

Re: Joint motion problem

TrippyLighting — Sat, 16 Jan 2016 02:06:57 GMT

If your project is a single file/document then you can simply export it as a .f3d file and attach that to your next post.

If some of the components are linked components (they have a little chain symbol in the browser), then its best to share a publich link. Here is a post that explains how to do that.

Re: Joint motion problem

Anonymous — Sat, 16 Jan 2016 04:10:00 GMT

Here's the .f3d file and thanks for looking at it.

Re: Joint motion problem

TrippyLighting — Sat, 16 Jan 2016 11:34:12 GMT

Thanks for sharing the model.

There are two items in the timeleine highlighted in yellow that you did not fix when they occured.

In this case they indicate that Fusion 360 is working with cached geometry and the original items in the timeline that created the geometry were deleted.

At some point in time during the creation process you received the message in the screenshot below and did not do anthing about it but continued with the process. You'll see tis when deleting the first joint in the timeline. That is likely the reason your joints don't work as expected. I'd roll the timeline back to before the first yellow icon, delete everything after it and re-create from there.

I also noticed hat you created that first Shoulder joint before you actually grounded the Axle and Cradle components. Not a good idea 😉

Re: Joint motion problem

Anonymous — Sun, 17 Jan 2016 16:53:30 GMT

Success! It all works properly now. Although, I have to say that I don't understand why.

I did as you suggested and recreated the components that were giving rise to the yellow warnings. Then I recreated the joints and tried to move the components. They were locked up again! It was similar to what happened before (i.e., suppressing one of the revolute joints freed up everything) but it wasn't exactly the same. When I edited the limits of the slider joint and hit OK, it all the components snapped into the proper relationship and then locked up. Finall, put in a zero value for the "rest" position of the slider and, suddenly, it all worked.

So, as I said, I have no idea what happened but it's all good now and I greatly appreciate your help. I've attached the .f3d file in case you want to dig around in it.

Re: Joint motion problem

TrippyLighting — Sun, 17 Jan 2016 16:55:55 GMT

Great. In that case I'll accept my answer as the solution and kudos to you

Re: Joint motion problem

Anonymous — Tue, 08 Mar 2016 19:32:19 GMT

I am having trouble of a somewhat similar nature although much simpler. I don't know if it's a mac problem or that I just don't understand joining two parts together in a revolute joint and then joining a third part to the second part and getting it to travel along with the second part. Take a look at the screen cast and see if you can figure it out. Thanks

Re: Joint motion problem

TrippyLighting — Tue, 08 Mar 2016 21:34:57 GMT

What screencast ?

Re: Joint motion problem

Anonymous — Wed, 09 Mar 2016 01:45:44 GMT

Sorry, I thought I had the link but I guess not. Here it is:

Re: Joint motion problem

Anonymous — Wed, 09 Mar 2016 01:48:17 GMT

Well, I go to my screencasts, grab the URL that they give me, come back here, reply and post that URL into the form that says: Screencast URL. Nothing shows up!

Re: Joint motion problem

Anonymous — Wed, 09 Mar 2016 01:50:09 GMT

Well, I go to my screencasts, grab the URL that they give me, come back here, reply and post that URL into the form that says: Screencast URL. Nothing shows up!

Re: Joint motion problem

TrippyLighting — Wed, 09 Mar 2016 02:00:33 GMT

PLease share your design. Export as .f3d and attach to your next post.

Re: Joint motion problem

Anonymous — Wed, 09 Mar 2016 04:01:55 GMT

Re: Joint motion problem

TrippyLighting — Wed, 09 Mar 2016 11:14:02 GMT

I have absolutely no problems creating a rigid joint between the cemter of the upper surface of the yellow pedal and the center of te inner, upper surface of the Link.

http://autode.sk/1R67eJY

Are you working with a mackbook pro and the trackpad ?

It almost seems that navigating Fusion 360 with the trackpad is limiting your options somehow.

Re: Joint motion problem

Anonymous — Wed, 09 Mar 2016 13:54:49 GMT

I'm using a MacBook Pro with a Logitech Track Ball. When I hover over any surface and hit Command, nothing at all happens. I would tend to think that is a Mac version problem, independent of the type of input device. But, that's not the real issue that I am having. I can work around that. What I don't understand is when you have three components, A, B and C and you make a revolute joint between A and B and a slider joint between B and C and you animate the A/B joint, why does C just stay where it is? Shouldn't it move with B? I can get it to move using a motion link but I don't necessarily want C sliding all of the time. I want the ability to slide C but not be forced to do so.

Re: Joint motion problem

TrippyLighting — Thu, 10 Mar 2016 00:20:34 GMT

No, the animation you see when creating the joint is ony between the two components that you are joining at that moment and it serves as visual feedback what degrees of freedome that joint allows.

It does not animate the complete assembly.

At this time not even the Animation workspace in Fusion 360 does not honor the joints. Hopefuly that gets corrected soon.

Re: Joint motion problem

Anonymous — Thu, 10 Mar 2016 00:39:53 GMT

Thank you. Sorry to hear that.

Re: Joint motion problem

jeff_strater — Thu, 10 Mar 2016 01:12:41 GMT

Just to be clear, there are several somewhat related things here:

First, Motion Link is an important concept. It lets you do some very simple relations between joints. Very useful for simulating gear relationships

1. the animation in the Joint commands. As @TrippyLighting says, this only shows a temporary animation between the immediate two components involved in the joint. We do this for performance reasons - if we tried to animate the entire chain of joints, the performance could potentially be bad

2. Animate Joint. This command just animates the selected joint only. It is much like the animation in the Joint command. ESC ends the animation

3. Animate Model. This command does solve the whole assembly, including Motion Links. Again, ESC ends the animation

4. Drive Joint. This command also solves the whole assembly, including Motion Links. The UI allows you to drag the degrees of freedom of the selected joints, to see how the assembly responds

5. Component drag. In this, you just drag a component, and it solves the entire assembly. You really need to have grounded at least one component in each joint chain for this to really work. And, this can be a bit wonky, since it is using view direction to determine the drag, but in general, this works well.

6. Motion Study. This command allows you to simulate a bunch of joints working together without having to use Motion Link. It is very powerful, but complex. And, you can only see it "play" from within this command.

Hoepfully, one of these will work for you.

Jeff Strater (Fusion development)