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Add geom adhesion: contacts that pull, via translated friction cones.

Browse Source 
https://youtu.be/GioWwB36XHI

The new geom attribute adhesion (units of force, signed; pair-level
override) translates the contact friction cone along its normal so
that the force origin lies strictly inside it. Consequences: each
contact can pull with up to the given force before breaking, and the
tangential friction budget becomes mu*(f_N + adhesion) -- the
Mohr-Coulomb yield condition with cohesion c = mu*adhesion -- so
lightly-squeezed grasps retain a guaranteed friction floor.

A translated cone factors exactly into {constant attractive force}
+ {original cone}, so no solver kernels change. The implementation is
this factorization: a constant attraction along contact normals
accumulated into the new mjData.qfrc_adhesion (summed into
qfrc_passive), plus a bias of adhesive contact rows' reference
acceleration (aref += R*adhesion), which makes resting penetration
exactly independent of adhesion. Contacts of adhesive pairs remain
active throughout the gap zone, producing rows with positive violation
whose reference acceleration pulls: a tether that resists pull-off
smoothly, captures objects released within the band into steady
contact, and detaches at the specified force. Adhesion values of the
two geoms combine by sum; explicit pairs override.

mj_contactForce reports the net interface force (cone force minus the
adhesive pull), whose normal component can now be negative. Negative
adhesion is allowed and produces a repulsive offset (air hockey).

PiperOrigin-RevId: 950858148
Change-Id: I879c08eba7ae501e5c0f8c2f807167344da4c2bc
This commit is contained in:
Yuval Tassa
2026-07-20 08:35:17 -07:00
committed by Copybara-Service
parent b942c9f922
commit a264d0bc8b
28 changed files with 784 additions and 50 deletions
Download Patch File Download Diff File Expand all files Collapse all files
src/user/user_model.cc
+4
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@@ -3063,6 +3063,7 @@ void mjCModel::CopyTree(mjModel* m) {
m->geom_margin[gid] = (mjtNum)pg->margin;
m->geom_gap[gid] = (mjtNum)pg->gap;
mjuu_copyvec(m->geom_surfacevel+6*gid, pg->surfacevel, 6);
m->geom_adhesion[gid] = (mjtNum)pg->adhesion;
mjuu_copyvec(m->geom_fluid+mjNFLUID*gid, pg->fluid, mjNFLUID);
mjuu_copyvec(m->geom_user+nuser_geom*gid, pg->get_userdata().data(), nuser_geom);
mjuu_copyvec(m->geom_rgba+4*gid, pg->rgba, 4);
@@ -3927,6 +3928,7 @@ void mjCModel::CopyObjects(mjModel* m) {
mjuu_copyvec(m->pair_solimp+mjNIMP*i, pairs_[i]->solimp, mjNIMP);
m->pair_margin[i] = (mjtNum)pairs_[i]->margin;
m->pair_gap[i] = (mjtNum)pairs_[i]->gap;
m->pair_adhesion[i] = (mjtNum)pairs_[i]->adhesion;
mjuu_copyvec(m->pair_friction+5*i, pairs_[i]->friction, 5);
}
@@ -5843,6 +5845,7 @@ bool mjCModel::CopyBack(const mjModel* m) {
pg->margin = (double)m->geom_margin[i];
pg->gap = (double)m->geom_gap[i];
mjuu_copyvec(pg->surfacevel, m->geom_surfacevel+6*i, 6);
pg->adhesion = (double)m->geom_adhesion[i];
if (nuser_geom) {
mjuu_copyvec(pg->userdata_.data(), m->geom_user + nuser_geom*i, nuser_geom);
@@ -5926,6 +5929,7 @@ bool mjCModel::CopyBack(const mjModel* m) {
mjuu_copyvec(pairs_[i]->solimp, m->pair_solimp+mjNIMP*i, mjNIMP);
pairs_[i]->margin = (double)m->pair_margin[i];
pairs_[i]->gap = (double)m->pair_gap[i];
pairs_[i]->adhesion = (double)m->pair_adhesion[i];
mjuu_copyvec(pairs_[i]->friction, m->pair_friction+5*i, 5);
}