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Physics constraint strange behavior

Hi everyone,

I have this strange problem with physics constraints. Attached is a simple project which demonstrates the problem. The scene contains 2 actors: a cable actor with a physics constraint component and a sphere static mesh actor. The sphere is attached to the cable actor via physics constraint so that the sphere "dangles" with a cable attached. My project also has the "degrees of freedom" set to YZPlane. LinearZ is limited to 100 and Swing 1 and 2 are locked. This works as expected, but when I set LinearZ to something larger like 200, the sphere begins to shake violently as in the attached screenshot. Does anyone know what is going on? Thank you.

alt text

sample project

Product Version: UE 4.19
clipboard01.png (334.1 kB)
myproject.zip (966.5 kB)
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asked Mar 20 '18 at 06:46 PM in Using UE4

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avatar image kevterm Mar 23 '18 at 04:04 AM

I just tried creating a project in 4.18.3 with the same setup and the problem goes away. Switching the project to 4.19 and the problem comes back! Looks like a bug in 4.19. Attached is the 4.18.3 project. MyProject for 4.18.3

myproject.zip (951.7 kB)
avatar image kevterm Mar 27 '18 at 01:33 AM

For anyone reading this, this problem becomes UE-56479.

avatar image kevterm Jul 22 '18 at 09:40 AM

Just tried the release version of 4.20 and the problem still exists.UE-56479 is marked "Won't Fix". I will have to stick to 4.18 for now. To anyone reading this: how do you guys achieve something similar to the sample project in 4.19/4.20? Thank you.

avatar image kevterm Oct 12 '18 at 09:44 AM

4.21P1 has the same problem. Can anyone help?

avatar image kevterm Nov 25 '18 at 11:56 AM

Modified project for 4.21. See post in Content Creation forum.

myproject.zip (974.9 kB)
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1 answer: sort voted first

Try putting the Physics Constraint in the world as a separate actor (superimposed with the cable actor) rather than attached/parented to the cable actor. It seems to fix the issue in your case (even if it's more like a workaround).

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answered Jan 29 '19 at 04:43 AM

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avatar image kevterm Jan 29 '19 at 06:15 PM

Thanks a lot for your reply, GeyserToo. I did a quick test as you suggested but the problem still seems to exist (project attached). The sphere on the right has a separate physics constraint actor while the sphere on the left was like before. Both spheres shake after the falling sphere touches them (see attached image). If you get it to work differently, do you mind uploading a sample project? Thank you.

myproject.zip (953.7 kB)
ue-56479.png (372.4 kB)
avatar image GeyserToo Jan 31 '19 at 05:11 PM

This is a completely different test from the 4.21 one (260389). In the 4.21 one you just had a sphere falling vertically and held back by a cable (limited along Z, locked along X and Y). Here you have two spheres that are apparently nudged by a falling third one and are (apparently?) supposed to start swinging after that. The single ball was clear enough, but here I no longer understand what you want to do. If you want to allow the spheres to swing, then why lock them along X and Y?

avatar image kevterm Feb 07 '19 at 11:14 AM

I should have replied to you, but I "add comment" instead by mistake. Please see below.

avatar image kevterm Feb 07 '19 at 10:08 AM

GeyserToo, I apologize for the confusion. Sample 266882 was just to show that the sphere still jitters even if I used a separate physics constraint actor. Besides, my game is a side-scroller and I had to put some physics limits; I should have removed them in the first place. Anyway, I made another sample in the past few days and this should be clearer of what I am trying to do.

I wish to simulate a crane lifting up objects. The scene has a hook mesh and the end has a physics constraint (and cable) component which constrains a cube below. Above the hook is a physics constraint actor (with cable) which constrains the hook itself. A level blueprint sets the Z-linear limit between 0 and 300 units according to time; this is how I simulate a crane lengthening or shortening the cable. The value of the limit is displayed on screen.

In 4.18, the scene works perfectly. Spheres are spawned upon mouse clicks and the cube behaves accordingly when hit by the spheres.4.18

In 4.21, the hook goes haywire when linear limit is > 200 and I have no idea of what I can do.4.21

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