[multibody] Add SapJointFrictionConstraint for joint dry friction - #24920
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[multibody] Add SapJointFrictionConstraint for joint dry friction#24920xuchenhan-lbm wants to merge 1 commit into
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Adds a single equation SAP constraint that models load independent dry (Coulomb) friction on one degree of freedom, the analog of MuJoCo's frictionloss. The cost is the Huber regularization of the maximum dissipation cost dt*tau_c*|v|, so the impulse is clamp(-v/R, +-dt*tau_c) with R = sigma*w, the same scale independent regularization SAP applies to contact friction. Friction saturates at the bound while sliding and holds the DOF at rest, up to a small regularization creep, in stiction. The constraint is not yet used by SapDriver; that follows separately.
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+a:@joemasterjohn for feature review please
@xuchen-han reviewed 4 files and all commit messages, and made 1 comment.
Reviewable status: LGTM missing from assignee joemasterjohn, needs platform reviewer assigned, needs at least two assigned reviewers, missing label for release notes (waiting on joemasterjohn).
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Towards #18932.
This PR adds
SapJointFrictionConstraint, a single equation SAP constraint that models load independent dry (Coulomb) friction on one degree of freedom joints. Future PRs will wire it up.Model
The constraint velocity is the joint velocity,
vc = eᵢᵀ v, and the friction impulse is bounded byγₘₐₓ = δt τ_c. The ideal maximum dissipation costγₘₐₓ |vc|is not differentiable atvc = 0, so, as for friction inSapFrictionConeConstraint, it is regularized. The cost is the Huber functionso the impulse
γ = −∂ℓ/∂vc = clamp(−vc/R, −γₘₐₓ, γₘₐₓ)is continuous and the Hessian is1/Rin stiction and zero in sliding. Friction always opposes the constraint velocity and|γ| ≤ γₘₐₓ, so the forceγ/δtis bounded byτ_c.The regularization is
R = σ w, withwthe diagonal Delassus approximation for the constraint (an estimate of the inverse effective inertia of the DOF) andσthe same dimensionless parameter used for contact friction. During stiction the DOF creeps with a residual velocity bounded byR γₘₐₓ = σ w δt τ_c, i.e.σtimes the velocity change friction alone could produce on the DOF in one step, and under a sustained loadτₐbelow the bound it settles atσ w δt τₐ.This change is