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fix(rne): propagate backward-pass force through the correct transform direction #691

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Merged
petercorke merged 1 commit into from
Sep 18, 2026
+11 −3
Merged

fix(rne): propagate backward-pass force through the correct transform direction #691

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2 changes: 1 addition & 1 deletion pyproject.toml
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Original file line number Diff line number Diff line change
Expand Up @@ -13,7 +13,7 @@ authors = [

dependencies = [
"numpy",
"spatialmath-python>=1.1.16",
"spatialmath-python>=1.1.18",
"spatialgeometry>=1.4.0",
"pgraph-python",
"scipy",
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10 changes: 9 additions & 1 deletion src/roboticstoolbox/robot/Robot.py
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Expand Up @@ -1928,7 +1928,15 @@ def rne(
i for i, group in enumerate(link_groups) if parent_idx in group
][0]

f[group_idx] = f[group_idx] + Xup[j] * f[j]
# Xup[j] is the child<-parent motion transform (v_child =
# Xup[j] * v_parent); propagating a force the other way,
# child->parent, needs the transform in the other
# direction too. spatialmath's SE3 * SpatialForce applies
# the coadjoint of its left operand (since
# spatialmath-python 1.1.18, rai-opensource/
# spatialmath-python#207), so the un-inverted transform
# (Xup[j].inv(), i.e. parent<-child) is what's needed here.
f[group_idx] = f[group_idx] + Xup[j].inv() * f[j]

# The current Q has the length equal to the number of links within the robot
# rather than the number of joints. We need to remove the static links
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2 changes: 1 addition & 1 deletion tests/test_ERobot.py
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Expand Up @@ -329,7 +329,7 @@ def test_symdyn(self):

self.assertEqual(
str(Q[0]),
"a1**2*m2*qd0**2*sin(q1)*cos(q1) + a1*qd0*(-a1*m2*qd0*cos(q1) - m2*r2*(qd0 + qd1))*sin(q1) - a1*(m2*(a1*qd0*qd1*cos(q1) - a1*qdd0*sin(q1) - g*sin(q0)*cos(q1) - g*sin(q1)*cos(q0)) + (qd0 + qd1)*(-a1*m2*qd0*cos(q1) - m2*r2*(qd0 + qd1)))*sin(q1) - a1*(-a1*m2*qd0*(-qd0 - qd1)*sin(q1) - m2*r2*(qdd0 + qdd1) + m2*(-a1*qd0*qd1*sin(q1) - a1*qdd0*cos(q1) + g*sin(q0)*sin(q1) - g*cos(q0)*cos(q1)))*cos(q1) + g*m1*r1*cos(q0) + m1*qdd0*r1**2 + m2*r2**2*(qdd0 + qdd1) - m2*r2*(-a1*qd0*qd1*sin(q1) - a1*qdd0*cos(q1) + g*sin(q0)*sin(q1) - g*cos(q0)*cos(q1))",
"a1**2*m2*qd0**2*sin(q1)*cos(q1) + a1*qd0*(-a1*m2*qd0*cos(q1) - m2*r2*(qd0 + qd1))*sin(q1) + g*m1*r1*cos(q0) + m1*qdd0*r1**2 + m2*r2**2*(qdd0 + qdd1) - m2*r2*(-a1*qd0*qd1*sin(q1) - a1*qdd0*cos(q1) + g*sin(q0)*sin(q1) - g*cos(q0)*cos(q1)) - (a1*sin(q1)**2 + a1*cos(q1)**2)*(m2*(a1*qd0*qd1*cos(q1) - a1*qdd0*sin(q1) - g*sin(q0)*cos(q1) - g*sin(q1)*cos(q0)) + (qd0 + qd1)*(-a1*m2*qd0*cos(q1) - m2*r2*(qd0 + qd1)))*sin(q1) - (a1*sin(q1)**2 + a1*cos(q1)**2)*(-a1*m2*qd0*(-qd0 - qd1)*sin(q1) - m2*r2*(qdd0 + qdd1) + m2*(-a1*qd0*qd1*sin(q1) - a1*qdd0*cos(q1) + g*sin(q0)*sin(q1) - g*cos(q0)*cos(q1)))*cos(q1)",
)
self.assertEqual(
str(Q[1]),
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