dartsim/dart — pull request #284 by jslee02 (2014-12-11 10:37:17+00:00)
Title: Consistent Jacobian and its derivatives of BallJoint and FreeJoint

This pull request change the computation of the Jacobians and Jacobian derivatives in consistent way. Previously, the generalized velocities and accelerations of `BallJoint` and `FreeJoint` were the spatial velocities and spatial accelerations, which are relative velocities and accelerations between the parent body node and the child body node, so their Jacobians and its derivatives were identity and zero matrices, respectively. The benefit of doing so is that the Jacobians and its derivatives don't need to be computed over simulation steps but the problem is that the velocities, accelerations, Jacobians and Jacobian derivatives are not consistent with the generalized coordinates.