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AI-driven physics scenes usually become unstable for a small set of engineering reasons: the integration step is too large, the constraint solver cannot converge, drives apply unrealistic forces, mass and inertia are wrong, colliders overlap, or the contact system loses collisions. The fastest reliable fix is to make the failure reproducible, record the actual timestep, then correct model and control errors before lowering fidelity or disabling collisions.
What an “exploding” AI physics simulation is telling you
An explosion is not a single error message. It is the visible result of an unstable numerical update: bodies acquire extreme velocity, joints stretch or spin, contacts generate huge impulses, or the solver repeatedly corrects the same impossible configuration. Jitter, objects passing through one another, excessive penetration, and robot joints that suddenly lock or launch are related symptoms.
Separate the policy from the simulator before changing parameters. A reinforcement-learning policy can issue actions that are physically unreasonable, but a bad mesh, zero inertia, or an incorrectly measured timestep can fail even when the same action sequence is replayed. The distinction matters because changing the policy will not repair a broken model, and increasing solver quality will not make an actuator command sensible.
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|---|---|---|
| Objects launch or gain energy rapidly | Large dt, excessive drive force, conflicting constraints, extreme mass ratio |
Replay fixed actions with learning disabled; halve the actual timestep |
| Persistent jitter at rest | Solver convergence, contact offset, overlapping colliders, stiff drives | Visualize contacts and compare PGS with TGS at the same timestep |
| Bodies pass through one another | Fast motion, insufficient contact capacity, unsuitable collision geometry | Enable CCD for the fast body and inspect dropped-contact warnings |
| Joint becomes unstable or oscillates | Drive stiffness, missing force or velocity limits, bad inertia, hard mimic joint | Clamp joint velocity and temporarily soften or disable the drive |
| Failure occurs on the first frame | Initial overlap, invalid mass or inertia, impossible joint limits | Start with collisions visible and inspect the initial transforms |
Start with a controlled, repeatable diagnosis
Do not tune a live learning run while the scene changes every episode. Save a minimal copy of the scene, a deterministic random seed, the exact asset versions, and the action sequence that produces the failure. Record the timestep passed to the physics step rather than relying on a nominal scene-frequency property.
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- Freeze learning updates. Replay the same observations and actions with policy updates, exploration noise, and randomization disabled. If the scene still fails, investigate the physics model first.
- Log the real integration interval. Capture the
dtvalue at every simulation step, including substeps or decimation between the controller and physics engine. A scene configured for a higher frequency does not necessarily change the interval your application passes to the step function. - Capture the first bad frame. Save positions, orientations, linear and angular velocities, applied forces, contacts, and joint errors immediately before and after the first visible divergence.
- Enable collision and contact visualization. Display contact points, normals, penetration depth, collider shapes, and filtered collision pairs. Temporarily disabling self-collision can identify a problematic link pair, but it is a diagnostic—not a permanent fix.
- Run a one-variable experiment. Change only
dt, solver iterations, drive limits, or collider geometry in each trial. Record stability, maximum penetration, joint error, and frame time so a change that merely hides the symptom is not mistaken for a repair. - Keep the smallest failing scene. Remove sensors, visual meshes, and unrelated agents while preserving the bodies, joints, and contacts involved. A minimal reproduction makes solver and asset errors much easier to isolate.
Fix timestep and numerical instability first
Integration approximates continuous motion with discrete updates. Complex mechanisms, closed kinematic loops, stiff contacts, and humanoid bodies have high natural frequencies; if the update interval is too large, the solver can overshoot a constraint before it has a chance to correct it. NVIDIA’s Omni Physics guidance uses 60 Hz as a common default reference, while humanoid simulations commonly run at 100 Hz or higher. Those are engineering reference points, not guarantees: the appropriate rate depends on the fastest motion, the smallest collider feature, and the stiffness of the mechanism.
Verify the timestep that is actually used
Changing a property labeled “frequency,” “rate,” or “updates per second” may alter scheduling without altering the duration passed to the simulation step. Log the value at the call site and calculate the corresponding rate as 1 / dt. Check controller decimation as well: a policy that acts every several physics steps can still produce unstable commands between policy updates.
Use a halving test
Run the same deterministic episode with dt reduced by half. If the failure disappears, the original step was outside the stable margin or exposed a stiff model that needs coordinated tuning. Keep the smaller step while you adjust solver iterations, drives, and constraints; then test whether a larger step can be restored without reintroducing instability. A smaller step increases computation, so measure runtime rather than assuming it is free.
Closed loops and humanoids need margin
Closed-loop mechanisms can impose several constraints on the same bodies at once. Their natural frequencies are often higher than a simple free body, so the default 60 Hz reference may be inadequate. Humanoid contacts and joint drives likewise tend to require 100 Hz or more in common configurations. Treat these figures as starting points and validate using maximum joint error, contact penetration, and energy behavior in your own scene.
Choose solver settings for convergence, not just speed
Projected Gauss-Seidel (PGS) and Temporal Gauss-Seidel (TGS) make different trade-offs. PGS generally converges more slowly but is less prone to overshooting. TGS can converge faster, yet complex collision configurations can overshoot and inject visible energy. Neither solver is universally correct; compare them on exactly the same scene, timestep, actions, and iteration budget.
| Choice | Typical behavior | When to try it | What to measure |
|---|---|---|---|
| PGS | Slower convergence with a tendency toward conservative corrections | Contacts or closed loops overshoot under TGS | Residual joint error, penetration, settling time, and frame time |
| TGS | Often faster convergence, but can overshoot in complex collision scenarios | You need speed and the scene remains stable at the selected timestep | Energy growth, contact impulses, bounce, and runtime |
| More position iterations | Improves constraint convergence at additional cost | Penetration or joint error remains after timestep and model checks | Maximum error and the cost per simulation step |
Smaller dt |
Widens the numerical stability margin but increases step count | Halving the timestep removes the failure | Stability improvement versus throughput loss |
Increase position iterations or reduce dt when the solver is not converging, but do not use iterations to compensate for invalid mass, overlapping colliders, or unbounded actuators. If a setting only works at an impractically high iteration count, the model is probably the real problem.
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Remove conflicts between hard constraints
Contacts, stiff joint drives, closed-loop joints, and mimic joints can all demand different positions at the same instant. Making every relationship perfectly rigid leaves the solver no physically plausible compromise. The result is oscillation, large corrective impulses, or a mechanism that appears to explode when two constraints become active together.
Make mimic relationships slightly compliant
For mimic joints, add compliance through a natural-frequency and damping-ratio specification instead of enforcing an infinitely hard relationship. NVIDIA’s PhysX guidance specifically recommends enough compliance for the mimic joint to yield just enough to avoid numerical issues. Tune compliance with the connected bodies and contact stiffness; an extremely soft mimic joint can introduce visible lag, while an infinitely rigid one can destabilize a closed loop.
Check the constraint graph
- Identify bodies connected by multiple joints, contact surfaces, or mimic rules.
- Verify that joint limits do not require a pose that another constraint forbids.
- Remove duplicate constraints and accidental parent-child cycles.
- Test the mechanism without external contact, then add contacts back one group at a time.
- Prefer a physically meaningful compliant connection over several contradictory hard constraints.
Keep drives and controllers within physical limits
A stiff drive can create a large force, acceleration, and velocity in one update, especially when the driven body is light or its inertia is wrong. In an AI loop, a policy that changes target positions sharply can repeatedly command those large corrections. Set a maximum drive force and, where the engine supports it, use actuator specifications rather than unconstrained idealized stiffness.
Inspect the lightest connected body
Drive settings that look reasonable for a heavy link can produce extreme acceleration on a small end link. Compare the commanded force with the mass and diagonal inertia of the body being accelerated. If the lightest body receives implausibly high acceleration, lower stiffness, add damping, cap force, or redesign the actuator model before increasing solver quality.
Limit velocity during early reinforcement learning
During early RL training, clamp revolute-joint velocity while the policy is still producing erratic actions. NVIDIA’s PhysX guidance gives 200–300 degrees per second as a generous range that often reduces instability. This is a training safeguard, not a universal operating limit; raise it only after the model, contacts, and controller remain stable at the intended motion range.
Distinguish control saturation from physics failure
Log target position, target velocity, measured velocity, applied force, and whether each limit was active. A joint that reaches its force limit and cannot track a target is a control-design issue. A joint that exceeds its limit, gains energy while the actuator is saturated, or becomes unstable with fixed actions points to a model, timestep, or solver issue.
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Repair mass, center of mass, and inertia
Every dynamic body needs nonzero, physically plausible mass, a sensible center of mass, a diagonal inertia tensor, and valid principal axes. Zero values, unit mistakes, or an inertia copied from the wrong scale can make a body effectively weightless in one direction or impossible to rotate. The resulting impulses and accelerations often look like a solver defect.
Use consistent units
Confirm the length, mass, and time units used by the asset importer, simulator, controller, and logged data. A model scaled by 1000 while its mass and inertia remain unchanged has radically different dynamics. Check that joint limits and velocity limits use the expected angle units; a degrees-versus-radians error can turn a normal command into an enormous one.
Avoid extreme ratios in connected bodies
Large mass or inertia ratios make contact and joint constraints harder to solve. Inspect adjacent links, not only the robot’s total mass. If one body is many orders of magnitude heavier or lighter than its neighbor, correct the physical data where possible. Do not add arbitrary mass merely to suppress jitter: that can hide an asset error and change the behavior the policy is learning.
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The center of mass should match the actual distribution of material, and the inertia axes should be aligned with the tensor representation expected by the engine. A center of mass far outside the mesh or an inertia tensor with implausible off-axis orientation can produce unexpected torque even when the visual model looks correct.
Eliminate collider overlap and unsuitable geometry
Initial collider overlap is one of the fastest ways to make a robot “explode.” The solver begins by generating a large repulsive impulse to separate bodies that already occupy the same space. NVIDIA’s Isaac Sim guidance warns that overlapping colliders can destabilize a simulation. Inspect the collision shapes—not just the render mesh—at the initial pose and after each imported transform.
Prefer simple collision shapes for moving parts
Use boxes, spheres, capsules, or convex hulls where they represent the shape adequately. Complex dynamic triangle meshes introduce more contact features and are harder to solve robustly. Replace a suspect mesh with a primitive or convex alternative and rerun the deterministic test. If stability returns, simplify or repair the original geometry rather than permanently accepting inaccurate contacts without measuring the effect.
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Filter contacts that should not exist
Many robot links are adjacent by design and should not collide with one another. Temporarily disable self-collision to locate the offending pair, then add a selective collision filter for links that are mechanically connected and never need contact. Keep legitimate self-collisions enabled elsewhere; globally disabling them can let bodies pass through one another and teaches an RL policy the wrong geometry.
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Repair signed-distance fields
For SDF-based collision, the source mesh must be watertight and well tessellated. A resolution that is too low can omit thin mesh sections because the grid cannot capture them. NVIDIA’s Omni Physics guidance reports that an SDF resolution around 250 often works well, while values above 1000 are rarely necessary. Increase resolution only when the geometry and scale justify it; an extremely fine SDF increases memory and preprocessing cost without correcting a non-watertight mesh.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Recover lost contacts, penetration, and tunneling
Contact failure has two opposite appearances: a body may tunnel through another at high speed, or it may remain deeply penetrated because the contact system did not retain enough constraints. Treat both as data-capacity and geometry problems before simply increasing stiffness.
Increase GPU contact capacity when contacts are dropped
GPU dynamics use bounded buffers for contacts. If warnings indicate that the maximum contacts per frame was reached, increase that capacity and rerun the scene. A larger buffer helps only when contacts are actually being discarded; it will not repair bad mass properties or overlapping initial poses.
Adjust contact offset deliberately
Contact offset determines how early the engine begins treating nearby shapes as contacting. An unsuitable value can produce visible gaps, late detection, or excessive correction. Change it with the same deterministic scene and inspect both penetration and resting separation. Record the scale of the objects because an offset appropriate for a small mechanism may be inappropriate for a large environment.
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Discrete collision checks can miss a thin object when it travels farther than its thickness between updates. Enable CCD for high-speed bodies when the engine and scene support it, then verify the additional runtime cost. CCD is not a substitute for an oversized timestep; if tunneling remains, reduce dt after correcting contact capacity and geometry.
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A complete fix sequence you can repeat
- Save a minimal scene and deterministic seed. Preserve the exact asset transforms, controller settings, solver choice, and action trace.
- Record the actual step interval. Log every
dtpassed to the physics step and any controller decimation. - Disable learning updates. Replay fixed actions to separate policy behavior from physics and asset behavior.
- Run the half-
dttest. If the failure vanishes, tune timestep, solver iterations, and drive natural frequency together. - Audit physical data. Check mass, inertia, center of mass, principal axes, joint limits, actuator force limits, velocity limits, and units for zeros or extreme ratios.
- Visualize contacts and colliders. Locate initial overlaps, excessive penetration, unexpected self-collision, and missing contact pairs.
- Simplify geometry. Replace complex dynamic meshes with primitives or convex hulls; repair watertightness before changing SDF resolution.
- Address contact capacity and speed. Increase GPU contact capacity when buffers are exhausted and try CCD for fast bodies.
- Compare PGS and TGS. Use identical conditions and retain the simplest solver configuration that meets the stability and accuracy target.
- Reintroduce complexity gradually. Restore learning, randomization, additional agents, and detailed colliders one change at a time, keeping the first failing configuration.
How to compare competing fixes
| Fix | Stability margin | Contact accuracy | Runtime cost | Risk of hiding a modeling error |
|---|---|---|---|---|
| Correct units, mass, and inertia | Usually high when data is wrong | Improves physical plausibility | Little ongoing cost | Low |
| Replace mesh colliders with primitives or convex hulls | Often high for moving bodies | May reduce geometric detail | Usually lower than complex meshes | Medium if shape simplification is excessive |
Reduce dt |
High numerical benefit | Can improve contact timing | More simulation steps | Medium if used without fixing invalid data |
| Increase solver iterations | Can improve convergence | May reduce penetration | Higher per-step cost | Medium |
| Add drive force or velocity limits | Usually improves control safety | Does not repair bad geometry | Low | Low to medium |
| Disable self-collision globally | Can stop one immediate failure | Removes legitimate contacts | Lower contact workload | High |
| Enable CCD | High for fast thin-body tunneling | Improves swept collision detection | Additional computation | Low |
Prefer corrections to units, mass properties, colliders, and control limits before permanently lowering fidelity or disabling collisions. A workaround is acceptable only when its effect is measured against the physical behavior you need.
Targeted troubleshooting branches
The scene explodes on frame zero
Inspect initial collider overlap, zero or negative physical values, invalid joint limits, and transforms imported at the wrong scale. Start with all drives disabled and gravity enabled. If the bodies still separate violently, the initial geometry or physical data is the primary suspect.
It fails only when the robot touches the environment
Visualize the contact pair, check contact offset and filtering, and test a primitive replacement for both colliders. Compare PGS and TGS with the same dt. If the failure disappears when self-collision is disabled, add a selective filter rather than removing all robot-environment contacts.
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It fails only during rapid motion
Log the fastest body’s velocity and the distance traveled per step. Apply a velocity limit, test CCD, and reduce dt. Check whether the actuator is commanding force beyond its specification; a collision setting cannot compensate for an unbounded drive.
It becomes unstable only during learning
Replay the recorded actions without policy updates. If replay is stable, constrain exploration, action scaling, and target changes; if replay fails, repair the simulator or model. During early training, the 200–300 degrees-per-second revolute-joint range suggested in NVIDIA’s PhysX guidance can provide a useful safety ceiling while the policy learns.
It is stable but too slow
Measure before reverting changes. Compare PGS and TGS, reduce unnecessary mesh complexity, and increase the timestep only after the deterministic scene remains stable. If a larger timestep causes penetration or energy growth, keep the smaller step for the affected mechanism or use substepping selectively if your engine supports it.
Validation checklist before trusting results
- The exact
dtis logged and matches the intended frequency. - Mass, center of mass, diagonal inertia, principal axes, and units are nonzero and plausible.
- Actuator force and velocity limits are explicit, and no light body receives extreme acceleration.
- Initial colliders do not overlap unintentionally, and collision filters match the mechanical design.
- Dynamic bodies use suitable primitive or convex geometry; SDF meshes are watertight and adequately resolved.
- GPU contact capacity is large enough for the observed scene, with no dropped-contact warnings.
- Fast bodies have an appropriate CCD policy, and tunneling is tested at the intended speed.
- Solver choice and iteration count are justified by measured stability, penetration, accuracy, and runtime.
- The scene is stable with fixed actions before learning, randomization, and additional agents are re-enabled.
- Results are checked across representative masses, poses, contact configurations, and action sequences rather than one lucky episode.
A stable simulation is one that preserves the intended physical relationships under the full range of states your AI system will encounter—not merely one that stops visibly exploding after a parameter is loosened.
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