SimDynamics provides an interface to physics engine support with an experimental implementation for Bullet.
Several issues have to be concerned for the current version:
* Simox/VirtualRobot allows to build robot structures that are not optimal when using them with Bullet. E.g.
    - Multiple revolute (hinge) joints following each other without any 3d models or translations can cause invalid worldTransform computations. (ToDo Allow Generic6Dof joints in Simox, then there we can model them directly in bullet)
    - Center of mass (com) transformations: Bullet computes com poses for all involved models.
      Currently the VirtualRobot's collision models are used to create bullet objects and here the com transformations are considered.
      But com transformations for VisualiztaionModels are ignored right now.
      If they are different to the collision models or if there is no collision model but a visu model, the visualization poses in simox will be invalid.
    - Bullet computes the joint value / angle by determining the rotation of the worldposes of the connected btRigidBodies. There seems to be a bug when the rotation is at +/- PI. Avoid transformations between RobotNodes that include a rotation around 180 degrees.
* Bullet models joints with btHingeConstraints. We found out that these constraints are not hard ones, so there can be situations where the constraints are violated which means that the robot structure looks somehow broken. Usually the constraint solvers of bullet try to re-adjust all limbs according to their constraints but if that is not possible or if the impacting forces/torques are too high the result is not as expected.
    - This effect is part of the solver algorithm, so it cannot be totally eleminated. But we noticed that multiple succeeding joints without displacement/3d models makes this artefact worse.

The best results can be achieved with modeling the robot the following way:
* Avoid succeeding revolute joints without models. Do not use more than one joint between two models.
* Always use Visualization and Collision Models (COM issue)