"get_net_contact_forces()" of a RigidPrimView always returns zeros

[matters]

Using Isaac Sim 2022.2.1, I am just trying to get contact forces to check for collision. The asset that I am trying to check for contact forces is MyAsset.usd and has the following tree. It represents a gate that helps products to enter a room (from a belt-conveyor) by using guides (like a “V” shape where the open part is outside the room), called rails. Because other parts of the gate are not of interest, I only kept the rails.

Both the RailfLeft and RailfRight Xforms have Physics as shown in the figure below. The EntryGate and the BaseLink do not.

I import this asset using the following Python commands

prim_utils.create_prim(
    prim_path='/World/EntryGate_0',
    usd_path= ASSET_PATH + '/MyAsset.usd'
)

I then clone them and set up a RigidPrimView for one of the Rails, which is a rigid body.

cloner = GridCloner(spacing=8.0)
cloner.define_base_env("/World/Env")

view_gate = RigidPrimView(
            prim_paths_expr="/World/Env/EntryGate_*/RailLeft",
            name="railf_left_view",
            reset_xform_properties=False,
            track_contact_forces=True,
            prepare_contact_sensors=True,
)
view_gate.initialize()
view_gate.post_reset()

Later, I try to get the contact forces to check for collision with

view_gate.get_net_contact_forces()

However, it ALWAYS returns zeros, even when I am seeing them colliding in the simulation. All collisions work, but the report of contact forces does not.

I tried setting up the collision and rigid body APIs on it, but I wasn’t successful.

prim = prim_utils.get_prim_at_path('/World/Env/EntryGate_0/RailLeft')

UsdPhysics.RigidBodyAPI.Apply(prim)
UsdPhysics.CollisionAPI.Apply(prim)

Can you explain all the requirements for the get_net_contact_forces() to work? It seems that just doing the obvious is not enough.

[Simplychenable]

@matters just curious as another user, are you experiencing this behavior in the latest Isaac Sim version as well?

[matters]

@Simplychenable I am not able to test it at the moment, I need to stay in this version otherwise I will have to refactor all the code and debug. The thing is, another asset is detecting the contact forces, but I didn’t make the usd file. I’m trying to check the difference, but without success. So that is why I wanted to know what is required for the contact forces to work, because the usual stuff is not working.