CODEWITHSUNDEEP
drake
Arjun Badyal
Arjun Badyal

Reputation: 25

Question about the composite body algorithm implementation in Drake

I am trying to relate algorithm 9.3 in Jain to the implementation of the composite body algorithm in Drake.

The documentation mentions that the hinge matrix is the transpose of that used in Jain. Looking in Featherstone (2008), it seemed this implementation was more in the spirit of section 6.3 of featherstone, i.e. S*=H, and the matrix is calculated as columns rather than blockwise. I wanted to double check if this was the case?

Upvotes: 1

Views: 99

Answers (1)

Sherm
Sherm

Reputation: 678

Yes, that's right. Although we use Jain's formulation and terminology (mostly) we like to think of the joint motion matrix as a small Jacobian as Featherstone does with his "S" (∂V/∂v, i.e. partial of spatial velocity V across the joint w.r.t. generalized velocities v of that joint). That way our H matrix follows the same orientation convention as does our full robotics Jacobian (where J maps velocites and Jᵀ maps forces).

We don't necessarily write down the H matrix for a given joint but rather have that joint (technically, mobilizer) provide functions for multiplying by H and Hᵀ.

Upvotes: 1

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