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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 resulting from gravity

Example:

import PyKDL as kdl

# Given: kdl_tree, base_link, end_link

chain = kdl_tree.getChain(base_link, end_link)
# 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/

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 resulting from gravityneeded to counteract the effects of gravity.

Example:

import PyKDL as kdl

# Given: kdl_tree, base_link, end_link

chain = kdl_tree.getChain(base_link, end_link)
# 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/