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Physics — Moving Parts and Payload

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Physics is off by default. Switch it on under Settings → Simulation → Physics (whole scene); it takes effect on the next model load.

With physics off, conveyors alone move the parts. Parts follow the belt, queue up and stop at the end of a line. With physics on, a part that leaves a conveyor becomes a physical body. It falls, slides down a chute, tips over and stacks in a bin.

Moving machine parts can push these bodies, and the parts they carry contribute to the load on the drive.

Moving parts push, lift and carry

Section titled “Moving parts push, lift and carry”

Any part the twin moves itself takes part in physics as soon as physics is on. This includes parts on a drive axis and links of a mechanism. A pusher pushes a box off the line. A lift table lifts what stands on it. A gripper pad carries a part from one station to the next, and lets go when it opens.

A moving part needs a collider. Author a Box Collider or a Mesh Collider on it in Unity. Unlike fixed scenery, a moving part has no automatic fallback to its bounding box. An estimated box on a gripper finger could misrepresent its contact geometry without warning, so parts without colliders stay passive.

What the two collider types become:

AuthoredWhat the physics sees
Box ColliderThe box, with its offset, rotation and scale as authored.
Mesh ColliderThe convex hull of the shape, like shrink-wrap around it. Pockets, slots and forks are filled in.

A hull is used even when the mesh is authored as concave; the message log says so. If a fork or a pocket has to grip something, model that region as its own box collider rather than relying on the mesh.

Colliders marked disabled or is trigger in Unity stay out of the physics, exactly as elsewhere.

The payload shows up in the drive forces

Section titled “The payload shows up in the drive forces”

When a mechanism link carries a part, its weight is measured at the contact and used in the mechanism’s force analysis. Open Quick Edit → Mechanism → Forces while the machine is running to see drive figures for the loaded machine. The peak and RMS values used for motor sizing include the part.

An extra column, Ext. load, appears in the same table as soon as any link carries something. It shows the force in newtons measured on that link in the previous simulation step. The column disappears when no link carries anything, rather than displaying dashes.

Contact loads reach force analysis on the next tick In tick n, drives and joints set the poses of kinematic, position-driven bodies. Moving colliders contact physical parts, the payload, in Rapier. After its step, Rapier measures the contact load. Only parts owned by a mechanism link report a load. In tick n+1, the mechanism's force analysis uses that load, one simulation step old, for Ext. load in newtons, drive force and torque, bearing loads, peak and RMS. The crossed return arrow shows that contact loads do not change poses. tick n tick n+1 Drives & joints Poses kinematic, position-driven bodies Moving colliders Rapier contact with payload physical parts after its step Contact load mechanism link ownership only does not change poses Quick Edit → Mechanism → Forces Load from tick n: one simulation step old Force analysis Ext. load (N) drive force/torque bearing loads peak and RMS
You read the payload in Quick Edit → Mechanism → Forces one simulation step after Rapier measures it. Only parts owned by a mechanism link report a load; the load never changes the poses.

Part weight. A part weighs 1 kg by default. Set a different weight per zone, or per part through the part’s PhysicsMassKg field. Without a specified weight, the physics engine would assign about 8 grams to a 200 mm cube, making payload figures unreliable.

What this does not do yet

Section titled “What this does not do yet”

Account for these limitations before using the figures to select a drive:

  • Friction from stacked parts is not passed down. If a part rests on another part, its weight reaches the link correctly. The friction it exerts on the part beneath it does not travel further down. A sliding stack therefore under-reports the sideways load.
  • A tipping part’s spin is ignored. Its contact forces and their lever arms are counted; the rotational inertia of the tipping itself is not.
  • The load is one simulation step old. Physics runs after the mechanism each step. A load measured now reaches the force analysis on the next step. This delay is invisible at 60 steps per second, but a hard impact appears one step late.
  • Parts that drive into a zone later. If the model uses explicit physics zones, each machine part is assigned to a zone when the model loads. A part that only later travels into another zone will not collide there. This limitation does not apply to the default whole-scene zone.
  • Physics never moves the mechanism. A mechanism always follows its drives and joints. A load can raise the force a drive has to produce; it can never bend, stall or drag the machine. The same motion without any parts produces exactly the same poses.

Checking that it is working

Section titled “Checking that it is working”

The Simulation settings tab shows a small diagnostics line while physics is running, for example:

2 zones / 17 bodies (4 kinematic) / 0.4 ms step

Bodies are the free parts; kinematic are the moving machine parts. If a gripper visibly travels through a part, check the kinematic count first. No kinematic bodies means no colliders were authored on the moving parts. In that case, the parenthesis is absent.

See also

Section titled “See also”