FA-87862 / Inverse kinematics solvers / Member archive

Shoulder angle adds the forearm offset instead of subtracting it · case 02

The shoulder points past the target by twice the forearm offset angle.

Member previewVariant 2 · 3 implementations · 7 checks per implementation

Case contract

Input [l1,l2,px,py,elbow] with positive link lengths. Return [q1,q2] in degrees (4 decimals) for a planar two-link arm whose end effector reaches (px,py); elbow "up" gives q2>=0, "down" gives q2<=0; q1 is wrapped to [-180,180). Points outside the annulus |l1-l2|<=r<=l1+l2 (1e-9 slack on r squared) return "unreachable".

Why this case matters

Inverse kinematics code turns task-space goals into joint commands; a wrong branch, sign, limit or update order sends a real arm to the wrong pose.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
general elbow down	`[0.0, -90.0]`	`[53.1301, -90.0]`	Failed

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