Modelling rigid body systems with closed kinematic loops

[NickoPen]

Hi,

1st post - new bullet user, although I have used other engines and written my own rigid body dynamics engine.

I have a test mechanism that I use to evaluate the accuracy of an engine's handling of rigid body mechanisms with closed kinematic loops. I am not sure what I am doing wrong, but my implementation in bullet fails miserably and it looks like there is too much compliance in the joints - would anyone be willing to look at my code and let me know what I am doing wrong?

I have the same mechanism implemented in ODE for comparison if that helps (which runs flawlessly, with reasonable accuracy), but the API between ODE and Bullet are quite different, especially the coordinate frames used for specifying joints.

The mechanism is a 6-linkage mechanism, similar to a 4-bar, but spatial in nature. All joints are simple 1-DOF hinges and it is actuated by a single torque.

Any offers to help would be appreciated!

[NickoPen]

No suggestions?

Is the silence telling me that Bullet doesn't have the ability to simulate a simple closed-loop system? Or is there a faulty articulated body constraint implementation?

[Kirity]

Hello NickoPen

Were you able to do it in Bullet ?? I do have same situation !

Any suggestion regarding closed kinematic loops w.r.t to Bullet and ODE are welcome

[Erwin Coumans]

Bullet should be at least as good, or better than ODE to deal with closed kinematic loops, either using btRigidBody or using btMultiBody. I'll create an issue to re-create an example to show this.

https://github.com/bulletphysics/bullet3/issues/384

[IresomPCH]

Hey, I've tried to build a 4-bar linkage in bullet simulation here:
http://bulletphysics.org/Bullet/phpBB3/ ... 103#p35103

The structure is set up in normal way using several RigidBody and HingeConstraint.
I encounter some stability issues at first, however the simulation becomes satisfied after using the double precision build.
Anyway, it's possible to build loop structure in bullet.

[Kirity]

@Erwin Coumans : Thanks for your reply !

@IresomPCH : Thanks for your inputs. This will help me !