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Fix sign error in flex interp implicit integration.

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The flex interp stiffness matrix is Negative Semi-Definite (NSD). When forming the RHS for implicit integration, the term involving the velocity and stiffness should be added, not subtracted. A new test is added to ensure energy stability for flex interp stretch stiffness with the implicitfast integrator.

PiperOrigin-RevId: 914845245
Change-Id: Iaaf0914909128e64e195f17cc5f2f344a8a43bc2
This commit is contained in:
Alessio Quaglino
2026-05-13 07:14:36 -07:00
committed by Copybara-Service
parent 69a1087e9e
commit 955ae3f3c0
3 changed files with 54 additions and 8 deletions
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src/engine/engine_forward.c
+2 -4
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@@ -1417,10 +1417,8 @@ static void flexInterp_cgsolve(const mjModel* m, mjData* d,
// build RHS: rhs = qfrc
mju_copy(rhs, qfrc, nv);
// flex_interp velocity correction: rhs -= h*K_interp*qvel
mju_zero(temp, nv);
mjd_flexInterp_mul(m, d, temp, d->qvel, h, 0); // temp = h*K_interp*v
mju_addToScl(rhs, temp, -1.0, nv); // rhs -= h*K_interp*v
// flex_interp velocity correction: rhs += h*K_interp*qvel (K_interp is NSD)
mjd_flexInterp_mul(m, d, rhs, d->qvel, h, 0); // rhs += h*K_interp*v
// standard flex bending velocity correction: rhs -= h*K_bend*qvel
mjd_flexBend_mul(m, d, rhs, d->qvel, -h, 0); // rhs -= h*K_bend*v