FA-88215 / Inverse kinematics solvers / Member archive

Negative elbow angles are treated as singular · case 05

Every elbow-down pose is damped as if it sat on the singularity.

Member previewVariant 5 · 3 implementations · 7 checks per implementation

Case contract

Input [l1,l2,q1,q2,vx,vy,w0,lam0]: resolved-rate step for a planar 2R arm. Manipulability w=|l1 l2 sin q2|. Damping lam^2 = 0 when w>=w0, else lam0^2 (1-w/w0)^2. Solve (J^T J + lam^2 I) qdot = J^T v; if that matrix determinant is below 1e-12 return ["singular", w]. Return [status, qdot1, qdot2] in degrees per second, status "damped" when lam^2>0 else "exact".

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
negative elbow near singular	`["damped", 0.3661, 6.7816]`	`["damped", -5.2725, 18.1241]`	Failed

MEMBER ARCHIVE

The complete case is available to members.

This record includes three runnable implementations, regression fixtures, execution results, and source hashes.

Member access is invitation-based. Sign in with your invited account to inspect the sources.