# Get Arm Gravity Compensation Joint Efforts from URDF, Joint Positions, and Grav Vector?

Given:

• a URDF
• the current joint positions
• a gravity vector

I want to calculate the joint efforts needed to compensate for gravity ("achieve weightlessness").

Is there an existing (ROS) package that will do this (preferably Python b/c existing code)?

I've been searching but haven't been able to find anything yet. Best lead (I think) is https://github.com/orocos/orocos_kinematics_dynamics/tree/master/python_orocos_kdl.

Originally posted by josephcoombe on ROS Answers with karma: 697 on 2018-03-28

Post score: 0

Comment by gvdhoorn on 2018-03-28:
It's not a ROS package, but would RBDL work for you? According to it's documentation:

There are multiple ways of creating Models for RBDL:

Comment by gvdhoorn on 2018-03-28:
KDL should also be able to do this I believe, but I haven't used it myself for this particular purpose.

Comment by josephcoombe on 2018-04-05:
@gvdhoorn Thanks for recommendations. After your suggestion, I looked into RBDL a little bit, but ended up going with KDL (since it's URDF support seems more mature/tested). PyKDL ended up having what I needed.

I figured out how to calculate joint efforts from gravity using KDL's Python API.

The kdl.ChainDynParam class provides a JntToGravity method which will produce the joint torques needed to counteract the effects of gravity.

Example:

import PyKDL as kdl

# Assuming: chain contains 3 rotational joints

grav_vector = kdl.Vector(0, 0, -9.81)  # relative to kdl chain base link
dyn_kdl = kdl.ChainDynParam(chain, grav_vector)
jt_positions = kdl.JntArray(3)
jt_positions[0] = 0.0
jt_positions[1] = 0.0
jt_positions[2] = 0.0

grav_matrix = kdl.JntArray(3)
dyn_kdl.JntToGravity(jt_positions, grav_matrix)

gravity_compensating_jt_torques = [grav_matrix[i] for i in range(grav_matrix.rows())]


Orocos KDL repository: https://github.com/orocos/orocos_kinematics_dynamics
PyKDL Documentation (incomplete): http://docs.ros.org/diamondback/api/kdl/html/python/

Originally posted by josephcoombe with karma: 697 on 2018-04-05

This answer was ACCEPTED on the original site

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