CODEWITHSUNDEEP
drake
jwelde
jwelde

Reputation: 25

Computing the generalized forces arising due to ExternallyAppliedSpatialForces in Drake, e.g. due to Propeller

I am working with a diagram which includes a MultiBodyPlant with a Propeller connected to it. The Propeller actually realizes numerous physical propellers which are distributed among the bodies of the MultiBodyPlant.

I am able to simulate the dynamics of the combined system by setting the prop forces with FixValue, so I'm on the right track.

What I'd like to be able to do is, given a configuration for the system (i.e. the MultiBodyPlant context) and a chosen propeller command, compute the generalized forces acting on the system. My sense is that this is not immediately available since the simulation is actually using the RNEA, and so does not aggregate the forces all together in that way. For what I'm doing (and even just as a sanity check), I would like to compute the forces directly, not just their effect on the evolution of the state.

Is there an existing method to compute this built into Drake, or should I compute it manually using the spatial jacobian of each propeller frame and the applied SpatialForce of the corresponding propeller? (Something along the lines of this question: How to get the matrix that maps external forces to generalized forces?)

Many thanks for your help.

Upvotes: 2

Views: 130

Answers (2)

Russ Tedrake
Russ Tedrake

Reputation: 5533

I understand better now. It's a very reasonable request! You have two systems at play: the Propeller and MultibodyPlant. Unfortunately, the quantity you want is all of Propeller and just a piece of MultibodyPlant. We don't offer direct access to the B(q) matrix in that case.

What you can do is call with either AutoDiffXd or symbolic::Expression, call CalcImplicitTimeDerivativesResidual on the Diagram to get the entire dynamics in implicit form (to avoid taking M(q) inverse). You could call it twice -- once with the Propeller inputs set up via FixValue as AutoDiffXd and/or symbolic::Variable and again with them as zero, then subtract the difference.

Note: CalcImplicitTimeDerivativesResidual is relatively new; I haven't pushed the python bindings for it yet (but it's been on my list). Do you need it from python?

Upvotes: 1

Russ Tedrake
Russ Tedrake

Reputation: 5533

I think that perhaps you are looking for the MultibodyPlant reaction_forces output port?

Upvotes: 0

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