API for extracting generalized inertia, velocity, contact Jacobians from Gazebo

osrf-migration commented 10 years ago

Original report (archived issue) by John Hsu (Bitbucket: hsu, GitHub: hsu).

osrf-migration commented 10 years ago

Original comment by Isura (Bitbucket: isura).

Which physics engines do you plan to use to support this?

osrf-migration commented 10 years ago

Original comment by John Hsu (Bitbucket: hsu, GitHub: hsu).

set assignee_account_id to "557058:f3968cd3-4910-4384-8349-482a6c7889ec"
set assignee to "hsu (Bitbucket: hsu, GitHub: hsu)"

osrf-migration commented 10 years ago

Original comment by John Hsu (Bitbucket: hsu, GitHub: hsu).

A rough draft of propose API for gazebo::physics:Model class functions:

GetIndependentVelocity(const Pose& P, Vector& v)  // get the velocities of the m degrees-of-freedom of the model (for floating bases, velocity is computed in frame P)

GetIndependentForces(const Pose& P, Vector& f)  // get the forces of the m degrees-of-freedom of the model (for floating bases, force is computed in frame P); forces will include gravity and centrifugal/Coriolis forces

GetIndependentInertia(const Pose& P, Matrix& m)  // get the m x m inertia matrix of the model (for floating bases, inertia is computed in frame P)

GetJacobian(math::Vector3& p, Link startLink, Link endEffector, const Pose& P, Matrix& m) // gets the Jacobian from the start link to the end-effector with respect to point p and defined in pose P

ComputeInverseDynamics(const Vector& qdd, Vector& tau) // computes the inverse dynamics (solves for tau) of the robot under gravity, centrifugal/Coriolis forces, and contact forces such that the robot's acceleration at the joints will be qdd

where

Layout of independent coordinates (assuming n joint degrees-of-freedom + 6 DOF base = m DOF total)

Each Jacobian (J) is a 6xq matrix, where q is determined by a chain specified by a startLink and an endEffector.

A contact Jacobian would be composed by multiplying [ n s t; zeros(3)] * J, where n, s, and t are the contact normal and the two contact tangents.

@isura I think we'll add the function calls in gazebo first, and start with lowest hanging fruits.

osrf-migration commented 10 years ago

Original comment by Michael Sherman (Bitbucket: sherm1).

For max performance you may want to think of these matrices primarily as operators that are applied to vectors. So rather than M, think of M*v. That way if a matrix is sparse (common) or if a matrix-vector product can be computed in an efficient matrix-free manner (also common) then the underlying implementation has a shot at doing it efficiently. Mass matrix, inverse mass matrix, constraint Jacobian, and task Jacobians for multiple tasks would all likely benefit from being considered operators. Getting the full matrix is also very useful, esp. during development, but likely to be suboptimal. It is always possible to create the matrix from the operator, although a special-purpose method that known it is returning the full matrix can possibly do it faster.