ALOHA Mobile → MuJoCo → Ansys

Real2Sim2FEA: From Robot Task Data to Structural Load Spectra

A general method for turning a robot's recorded task data into load cases and cycle spectra for its own structure — then that method narrowed onto one bimanual ALOHA task, 557 episodes of it, and carried through to a solved upper-arm stress field.

557episodes

504,000samples

2.8 hrobot time

12joints extracted

12.69N·m peak

17.77MPa von Mises

Three photographs: a gamepad driving the arm by inverse kinematics, the Mobile ALOHA platform, and the tube-transfer bench — **Fig 1.** The rig. Gamepad-IK teleoperation drives the follower arms (a); the Mobile ALOHA platform the pipeline was written for (b); the tube-transfer bench this corpus was recorded on (c).

Filmstrip of one episode in six phases — start, grasp, lift, transport, insert, release — in overhead and wrist-camera views — **Fig 2.** One episode, six phases, overhead view above and wrist camera below. Five hundred and fifty-seven of these — 2.8 hours of a robot doing real work — are the entire input to everything that follows. No load cell, no test rig, no assumed duty cycle.

Three MuJoCo renders of the tube transfer: grasping at the source rack, the tube held aloft in transfer, and insertion into the destination rack — **Fig 3.** The same task in MuJoCo. The loads on this page come from this arm model with the props removed and contacts off, so only the arm's own weight and motion enter the wrench.

01 — every joint

What each joint actually carries

Inverse dynamics on the recorded poses gives every joint’s torque and every link’s interface wrench at once, so the FEA target can be chosen from the numbers rather than before them.

Cycle spectra

Rainflow exceedance — how many cycles reach at least a given amplitude. All twelve panels share log–log axes so horizontal position is comparable; grey curves behind each are the other eleven.

02 — the finite element model

Structural FEA Modeling

The peak instant applied to the upper arm as a free body loaded at both ends.

CAD model of the complete ViperX 300 arm assembly, parts colour-coded — **Fig 4.** The vendor assembly, 23 solid bodies. Every part maps one-for-one onto a MuJoCo link, as it must — the model descends from this CAD via the URDF.

The upper arm part isolated from the assembly — **Fig 5.** The upper arm isolated. Shoulder yoke at one end, elbow servo mounts at the other; 0.793 kg, 322.8 mm long.

Tetrahedral mesh on the upper arm — **Fig 6.** The refined mesh — elements now resolve the yoke plates and the fillets where the peak sits.

Ansys load application showing four remote forces, four moments and a remote displacement — **Fig 7.** Four remote points, two per interface, because a dual-servo joint introduces its load at two coaxial patterns rather than one.

Von Mises stress distribution on the upper arm, peak 17.77 MPa at the shoulder yoke — **Fig 8.** Equivalent (von Mises) stress under the peak task load.