Abhijeet Rao

DOM Workshop
IROS 2026

Abhijeet Rao · MPhil thesis research

Supervised and presented by Dr. Rika Antonova

University of Cambridge

Sunday 27 September 2026

Concave scoop · silicone fingertip

Co-designed silicone fingertips

We co-design our fingertips and make them in silicone.

The simulation gap

Deformable in reality. Rigid in our current simulation.

RealityDeformable silicone
SimulationRigid representation

Simplified high fidelity scenario

We study this in the highest fidelity possible using Abaqus as a benchmark.

Abaqus · pad and sphere

Deliberate coarse-graining

We remove the interior using a reduced-order modelling technique.

Pad cross-section
Operator

Result - 1 contact event

One pad. One sphere.

Native MuJoCo 3D Flex
Our Model
faster simulation
42×
lower force-output RMSE
56%
lower normal-deformation RMSE
42%

Result - 100 contact events

100 contacts Drag to rotate
faster simulation
9×
lower force-output RMSE
53%
lower normal-deformation RMSE
33%

Force and deformation RMSE measured against Abaqus.

Extension to arbitrary geometries

Drag to rotateSqueezeRun 42 · prescribed motion

Further implications
and applications

Towards scalable simulation for soft robotics.

FURTHER IMPLICATIONS

Can we predict
material deformation?

20 mN

FURTHER IMPLICATIONS

Significant mismatch

≈44%

lower tip displacement
in MuJoCo than the Abaqus reference.

MuJoCo13.85 mm
Abaqus24.63 mm

NATIVE MUJOCO 3D FLEX · OUR MODEL

MuJoCo build details

42× faster simulation, 56% lower force-output RMSE and 42% lower normal-deformation RMSE. Both models are evaluated against Abaqus over the same 50 single-sphere cases.

FROZEN ENGINE BUILD

MuJoCo 3.13.0

CG · discrete · float64 · Flex cap 800

Download settings

Release commit 123347c0eeab7e13c8da0828ab593bbd95bcf335

Solver and integration
Constraint solverConjugate Gradient (CG), both models
Integratordiscrete, both models
Timestep · native Flex23.75 µs
Timestep · our model80.46875 µs
CG iterations / tolerance100 / 1e-8
Line search iterations / tolerance50 / 0.01
Jacobian / friction coneauto / elliptic
Friction impedance ratioimpratio = 1
No-slip iterations / tolerance0 / 1e-6 (refinement inactive)
CCD iterations / tolerance35 / 1e-6
Warm start / reference safetywarmstart = enable
refsafe = enable
Energy reportingEnabled; same profile in both arms
Contact and friction
Contact dimensioncondim = 6
solref0.0002 1 (time constant in seconds; damping ratio)
solimp0.99 0.999 0.0001 0.5 2
Sliding / torsional / rolling friction0.8 / 0.005 / 0.0001
Friction unitsSliding is dimensionless; torsional and rolling coefficients are 0.005 m and 0.0001 m.
Effective contact friction vector0.8 0.8 0.005 0.0001 0.0001
Margin / gap0 / 0 m
Priority / mixing weightpriority = 0; solmix = 1
Flex collision radius1e-8 m
Flex self / internal collisionsselfcollide = none; internal = false
Flex passive contact / active layerspassive = false; activelayers = 1
Flex contact cap800 per Flex pair; local mjFLEXCONPAIR patch
Contact ownershipNative MuJoCo rigid–Flex contact in both arms; no custom contact-force injection
ApplicationIdentical settings on the rigid geom and each Flex; no explicit contact-pair overrides
Material, geometry and boundary conditions
Silicone density1100 kg/m³
Material cardC10 = 134000 Pa; D1 = 1e-7 Pa⁻¹
Shear / bulk moduli268000 Pa / 20000000 Pa
Native elastic mappingE = 800424.7693635096 Pa
ν = 0.49332979358863743
Extra dampingFlex edge and Rayleigh damping 0; joint damping, armature and friction loss 0
Pad50 × 50 × 10 mm; 42 × 42 × 9 hexahedral cells in the reference geometry
SupportBonded bottom: all bottom nodes fixed; top and sides free
Rigid sphereOriginal mesh, radius 0.006203504908994003 m; 2,402 surface vertices and 2,400 quadrilateral facets (triangulated for MuJoCo)
Sphere density / mass7850 kg/m³ / 0.00785 kg
Sphere inertiaIxx = Iyy = Izz = 1.2083810570956585e-7 kg·m²; off-diagonal terms zero
Native discretisation3D tetrahedral Flex from the reference hexahedra; fixed-first, reversed symmetric minimum-degree ordering
Our modelRetained contact surface; material forces through a passive callback
Arithmetic / unitsfloat64 physics and material/operator arithmetic; SI units
Gravity0 0 -9.81 m/s²
Fluid effectsDensity and viscosity 0; wind 0 0 0
Scoring and runtime measurement
CohortThe original 50 cases from 20260907_PAD_EXPERIMENTS_SINGLE_SPHERE; original initial poses, velocities and case durations
Force-output RMSEAll three world-frame contact-force components on the sphere, against Abaqus
Normal-deformation RMSEThe original 169 common moving top-surface nodes, against Abaqus
SamplingEvery solver increment over the whole window; force interval timestamps and state timestamps handled separately by the scoring bridge
SpeedupNative Flex physics runtime / our model’s physics runtime
Error reduction100 × (1 − our model’s RMSE / native Flex RMSE)
Physics timing includesmj_step1, mj_step2 and material callbacks
Physics timing excludesOffline operator assembly, runtime setup, recording, scoring and convergence diagnostics
ExecutionPinned single-thread workers, up to 50 in parallel on socket 1; separate fleet launches
Timing contextParallel-fleet timings; host and memory contention may differ between launches
Build identity and settings
Build profilemujoco-3.13.0-cap800-discrete-f64-v1 with CG and branch timestep overrides
PlatformLinux x86_64 · Python 3.12.3 · GCC 13.3.0 · CMake 3.28.3 · Release build
Shared-library SHA-256dff2563f072358f1008fe383088868bcc1e3ec0ed45aecd7a2c92c7b6fcec832
Patch SHA-2565af9bc6febf052c7a2866401e32de6bb339125146247ff62b62f36eeabcadd7c
All compiled numerical optionsDownload settings JSON
Effective XMLNative Flex options · Our model options
ProvenanceFrozen build verification and contact XML. The download includes their source hashes and the cap patch.
Full-run inputsOriginal meshes, operators, Abaqus histories and per-run receipts remain in the research repository; this download contains the displayed settings and build evidence.

DOM · IROS 2026

Scalable soft contact

Abhijeet Rao · MPhil thesis

All reported runtime and error reductions compare our model with Native MuJoCo 3D Flex. Both models’ force-output and normal-deformation RMSE are measured against Abaqus.

The looping impact uses the supplied Abaqus video. Its crop removes only the surrounding annotations. The coloured 3D geometry and living mesh are explanatory visuals, not additional solver results.

The two-dome mount motion follows batch run 42, with its motion-program hash verified against the saved batch record. Cube motion and local contact flattening are prescribed illustrations; they are not solved dynamics. The pneumatic-finger visual reuses the original saved geometry and Abaqus/MuJoCo displacement fields; intermediate shapes are illustrative. Original gripper design: Jessica Ganley, CC BY-NC-SA 4.0.

Scroll or use ← → / Page Up / Page Down. F enters fullscreen. The Abaqus slide starts on the first frame. Advance once to play its video. On the operator slide, advance once to reduce it to the retained surface. The expanded chapter dots show and advance these steps.