btMultiBodyJointLimitConstraint produces unstable multibody

[ryu]

Hi,

First of all, thanks a lot for the btMultiBody, I think it's a great feature. I have been playing with it, and found some strange behavior when I enable limit constraints. I attached a file based on FeatherstoneMultiBodyDemo.cpp that reproduces the error I have been seeing. It is a very simple multibody with just two pieces. Steps to reproduce:

1. Open the demo
2. Grab the object and make it fall on the side
Expected:
- Stays on the floor, still
Actual:
- It starts rotating randomly

Another test that can be done is to comment out the following line:

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  limitCons->setMaxAppliedImpulse(40);

In this case (I assume it uses a maxImpulse of 100), even though the constraint allows for 180 degrees of rotation, it stays fixed in one of the limits, and there is no way to make it turn.

I also reproduced the strange behavior by creating the whole scene in Z-UP mode (with gravity [0,0,-10]) instead of Y-UP, and in this case I don't even need to make it fall on the side, it keeps rotating while standing on the base of the cylinder (and eventually falls by itself).

Am I using the API wrong? Or is it a bug?

Also, if I may make a suggestion, please add an API to set link colliders, having to do the following is unintuitive, given that there is an API for the base:

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  multiBody->getLink(linkIndex).m_collider = linkCollider;

Thanks!

[benelot]

This problem seems to be resolved. At least I can not reproduce it by adding:

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		int linkIndex = 2;
	// Link joint limits
	btMultiBodyConstraint* limitCons = new btMultiBodyJointLimitConstraint(
		pMultiBody, linkIndex, -1.6, 1.6);
	// The default value (100) behaves like a lock on -1.6
//	limitCons->setMaxAppliedImpulse(40);
	pWorld->addMultiBodyConstraint(limitCons);

to the current MultiDoF demo. Somewhere after finalizeMultiBody in the method:

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btMultiBody* MultiDofDemo::createFeatherstoneMultiBody_testMultiDof(btMultiBodyDynamicsWorld *pWorld, int numLinks,const btVector3 &basePosition, const btVector3 &baseHalfExtents,const btVector3 &linkHalfExtents, bool spherical, bool floating)

[ryu]

Thanks!

I can't reproduce it anymore either. Seems resolved.
And in general, btMultiBody seems much more robust recently.

[benelot]

True, I have the same feeling that it is a lot more robust. However the proper control of a spherical joint similar to a btGeneric6DoFJoint does not work so great yet it seems. Also I am lacking the limits there. What a pity!