FA-87938 / Inverse kinematics solvers / Member archive

CCD sweep reuses tip position from before earlier joint updates · case 03

Joints nearer the base rotate toward a tip position that no longer exists, overshooting the target.

Member previewVariant 3 · 3 implementations · 7 checks per implementation

Case contract

Input [lengths, angles, target, iters, tol, lim]: planar serial chain with relative joint angles in degrees and a symmetric joint limit +/-lim degrees. Each iteration first stops if the tip is within tol of target, else counts one sweep that visits joints from the tip joint down to the base, re-evaluating forward kinematics before each joint, rotating by the signed angle between tip and target vectors and clamping the joint. Return [angles (3 decimals), sweeps performed, final error (4 decimals)].

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
target behind base	`[[-172.405, -121.707], 3, 1.3428]`	`[[-121.779, -101.253], 3, 0.2445]`	Failed

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

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