Stabilise the hands demo on MJWarp and command its tendons

The Shadow Hand's tendon-coupled fingers diverge past their joint limits under
the default explicit integrator, which corrupted memory and surfaced as a
SIGSEGV in an unrelated collision kernel. Measured over 260 steps against a
1.571 rad limit: euler reaches 3.275 with 15 joints out of range, implicitfast
0.944 with none, matching develop's 1.041.

The demo also never commanded the tendons, so the eight joints they span took
no command at all, and posed the hand with the reorientation task's spawn pose,
which left the palm facing down beside the Allegro.

Resolve the MuJoCo actuator rows of an articulation spawned outside a cloned
scene, where the importer leaves actuator_world at -1 and partitioning by world
selected nothing. Without it the tendon control never binds and commanding a
tendon reports the asset as authoring no tendon actuator.

Author: hujc7

scripts/demos/hands.py

@@ -54,6 +54,7 @@
 from isaaclab_assets.robots.shadow_hand import (
     SHADOW_HAND_NEWTON_CFG,
     SHADOW_HAND_PHYSX_CFG,
+    TENDON_POSITION_LIMITS,
 )
 
 if TYPE_CHECKING:
@@ -98,7 +99,14 @@ def design_scene() -> tuple[dict, list[list[float]]]:
     sim_utils.create_prim("/World/Origin2", "Xform", translation=origins[1])
     # -- Robot
     shadow_hand_cfg = SHADOW_HAND_NEWTON_CFG if args_cli.physics == "newton_mjwarp" else SHADOW_HAND_PHYSX_CFG
-    shadow_hand_cfg = shadow_hand_cfg.replace(prim_path="/World/Origin2/Robot")
+    # Pose for this side-by-side scene; the asset's own pose is the reorientation task's.
+    shadow_hand_cfg = shadow_hand_cfg.replace(
+        prim_path="/World/Origin2/Robot",
+        init_state=shadow_hand_cfg.init_state.replace(
+            pos=(0.0, 0.2, 0.5),
+            rot=(0.52296271, -0.47593067, 0.47593067, 0.52296271),
+        ),
+    )
     shadow_hand = shadow_hand_cfg.class_type(shadow_hand_cfg)
 
     # return the scene information
@@ -151,6 +159,15 @@ def run_simulator(sim: "sim_utils.SimulationContext", entities: dict[str, "Artic
             joint_pos_target = robot.data.soft_joint_pos_limits.torch[..., grasp_mode]
             # apply action to the robot
             robot.set_joint_position_target_index(target=joint_pos_target)
+            # A tendon has no actuator on its spanned joints, so it needs its own command.
+            # Span comes from the asset: a fixed tendon authors no position limit of its own.
+            if robot.num_fixed_tendons > 0:
+                tendon_pos_target = torch.full(
+                    (robot.num_instances, robot.num_fixed_tendons),
+                    TENDON_POSITION_LIMITS[grasp_mode],
+                    device=robot.device,
+                )
+                robot.set_fixed_tendon_position_target_index(target=tendon_pos_target)
             # write data to sim
             robot.write_data_to_sim()
         # perform step
@@ -168,6 +185,9 @@ def main():
     with launch_simulation(cfg=PhysicsCfg(), launcher_args=args_cli) as physics_cfg:
         # The default newton mjwarp solver configuration needs to be tuned for these hands.
         if isinstance(physics_cfg, NewtonCfg) and isinstance(physics_cfg.solver_cfg, MJWarpSolverCfg):
+            # The tendon-coupled fingers diverge past their limits under the default explicit
+            # integrator; the reorientation task's preset uses this one for the same reason.
+            physics_cfg.solver_cfg.integrator = "implicitfast"
             physics_cfg.solver_cfg.njmax = 200
             physics_cfg.solver_cfg.nconmax = 70
             physics_cfg.solver_cfg.impratio = 10.0

source/isaaclab_newton/isaaclab_newton/physics/_mjwarp_view_compat.py

@@ -200,6 +200,18 @@ def _actuator_rows_per_instance(view: ArticulationView, model: Model, tendon_cou
     tendon_worlds = _to_numpy(model.mujoco.tendon_world).astype(np.int64, copy=False)
     ordinal = _ordinal_within_world(view, tendon_count)
 
+    # An articulation spawned outside a cloned scene keeps ``actuator_world`` at the -1 the
+    # importer writes, so partitioning by world would select nothing and silently report the
+    # articulation as having no actuators. Every row belongs to this view's single world in that
+    # case -- MEASURED on the hands demo: 20 actuators all at -1 against tendons at world 0.
+    if actuator_worlds.size and int(actuator_worlds.max()) < 0:
+        if view.count != 1:
+            raise ValueError(
+                f"MuJoCo actuator rows carry no world assignment, so the {view.count} articulations"
+                " this view selects cannot be told apart."
+            )
+        return np.arange(actuator_worlds.size, dtype=np.int64).reshape(1, -1)
+
     per_instance = []
     for world in range(view.world_count):
         rows = np.flatnonzero(actuator_worlds == world)