Implicit Actuator doesn't work as Isaac Sim Joint Drive and the arm falls down

[arambarricalvoj]

Hi,

I'm starting to set up an environment for my robot. I've created the joints as ImplicitActuators with the same stiffness and damping values as in the manufacturer's USD file.

However, when I run this environment, it seems like the weight is overloading some of the joints, whereas in the Isaac Sim simulation (without Isaac Lab), the JointDrive works correctly, as shown in the video.

isaac_lab_implicitActuatorFalls.webm

I tried using IdealPDActuator, which does solve the problem, but the movement speed of some joints becomes very slow, and I haven’t found a way to adjust it.

Am I doing something wrong?

The .usd file used is an official version and provided by the manufacturer (although it has been modified only to disable the gripper). The code of my environment for Isaac Lab is as follows:

import argparse
from isaaclab.app import AppLauncher

parser = argparse.ArgumentParser()
parser.add_argument("--num_envs", type=int, default=4)
AppLauncher.add_app_launcher_args(parser)
args_cli = parser.parse_args()

app_launcher = AppLauncher(args_cli)
simulation_app = app_launcher.app

import torch
import isaaclab.sim as sim_utils
from isaaclab.scene import InteractiveScene, InteractiveSceneCfg
from isaaclab.assets.articulation import ArticulationCfg
from isaaclab.actuators import ImplicitActuatorCfg, IdealPDActuatorCfg
from isaaclab.assets import AssetBaseCfg
from isaaclab.utils import configclass

USD_PATH = "/isaac-sim/projects/ufactory_ros2/isaac-sim/scripts/javi_xarm6.usd"

# --- Robot config ---
XARM6_CONFIG = ArticulationCfg(
    prim_path="{ENV_REGEX_NS}/Robot",
    spawn=sim_utils.UsdFileCfg(usd_path=USD_PATH),
    init_state=ArticulationCfg.InitialStateCfg(
        pos=(0.0, 0.0, 0.1),
    ),
    actuators={
        "joint1": ImplicitActuatorCfg(
            joint_names_expr=["joint1"],
            stiffness=73.29828,
            damping=0.02932,
        ),
        "joint2": ImplicitActuatorCfg(
            joint_names_expr=["joint2"],
            stiffness=114.32549,
            damping=0.04573,
        ),
        "joint3": ImplicitActuatorCfg(
            joint_names_expr=["joint3"],
            stiffness=95.28044,
            damping=0.03811,
        ),
        "joint4": ImplicitActuatorCfg(
            joint_names_expr=["joint4"],
            stiffness=283.06,
            damping=0.11322,
        ),
        "joint5": ImplicitActuatorCfg(
            joint_names_expr=["joint5"],
            stiffness=9.7635,
            damping=0.00391,
        ),
        "joint6": ImplicitActuatorCfg(
            joint_names_expr=["joint6"],
            effort_limit_sim=20.0,
            stiffness=2.07821,
            damping=0.00083,
        ),
    }
)

# --- Scene config ---
@configclass
class Xarm6SceneCfg(InteractiveSceneCfg):
    ground = AssetBaseCfg(
        prim_path="/World/Ground",
        spawn=sim_utils.GroundPlaneCfg()
    )
    light = AssetBaseCfg(
        prim_path="/World/Light",
        spawn=sim_utils.DomeLightCfg(intensity=3000.0)
    )
    robot = XARM6_CONFIG


def run_sim(sim, scene):
    sim_dt = sim.get_physics_dt()
    sim_time = 0.0
    count = 0

    while simulation_app.is_running():

        if count % 700 == 0:
            print("[INFO] Resetting robots...")
            count = 0

            root = scene["robot"].data.default_root_state.clone()
            root[:, :3] += scene.env_origins
            scene["robot"].write_root_pose_to_sim(root[:, :7])
            scene["robot"].write_root_velocity_to_sim(root[:, 7:])

            jpos = scene["robot"].data.default_joint_pos.clone()
            jvel = scene["robot"].data.default_joint_vel.clone()
            scene["robot"].write_joint_state_to_sim(jpos, jvel)

            scene.reset()

        # ---------------------------------------------------------
        # CONTROL POR POSICIÓN: mover joint1 y joint3
        # ---------------------------------------------------------
        target_pos = scene["robot"].data.default_joint_pos.clone()

        # convertir sim_time → tensor
        t = torch.tensor(sim_time, device=target_pos.device)

        # joint1 → movimiento sinusoidal suave
        #target_pos[:, 0] = 0.7854 #1.5708 #0.5 * torch.sin(2 * torch.pi * 0.5 * t)

        target_pos[:, 1] = 0.0#-0.5236

        target_pos[:, 2] = -0.90#-0.5236

        # enviar comando
        scene["robot"].set_joint_position_target(target_pos)
        # ---------------------------------------------------------

        scene.write_data_to_sim()
        sim.step()
        scene.update(sim_dt)

        sim_time += sim_dt
        count += 1


def main():
    sim_cfg = sim_utils.SimulationCfg(device=args_cli.device)
    sim = sim_utils.SimulationContext(sim_cfg)
    sim.set_camera_view([3, 0, 2], [0, 0, 0.5])

    scene_cfg = Xarm6SceneCfg(num_envs=args_cli.num_envs, env_spacing=2.0)
    scene = InteractiveScene(scene_cfg)

    sim.reset()
    print(f"[INFO] Scene ready with {args_cli.num_envs} robots")

    run_sim(sim, scene)


if __name__ == "__main__":
    main()
    simulation_app.close()

[arambarricalvoj]

Solved by setting stiffness and damping to None, as it takes the values from USD (see the code below). However, it's strange that it doesn't work when setting the same values in USD. Why is this happening?

"joint1": ImplicitActuatorCfg(
            joint_names_expr=["joint1"],
            stiffness=None,#73.29828, #Con None no se modifica nada del USD.
            damping=None,#0.02932,
        ),
        "joint2": ImplicitActuatorCfg(
            joint_names_expr=["joint2"],
            stiffness=None,#114.32549,
            damping=None,#0.04573,
        ),
        "joint3": ImplicitActuatorCfg(
            joint_names_expr=["joint3"],
            stiffness=None,#95.28044,
            damping=None,#0.03811,
        ),
        "joint4": ImplicitActuatorCfg(
            joint_names_expr=["joint4"],
            stiffness=None,#283.06,
            damping=None,#0.11322,
        ),
        "joint5": ImplicitActuatorCfg(
            joint_names_expr=["joint5"],
            stiffness=None,#9.7635,
            damping=None,#0.00391,
        ),
        "joint6": ImplicitActuatorCfg(
            joint_names_expr=["joint6"],
            stiffness=None,#2.07821,
            damping=None,#0.00083,
        ),
        "drive_joint": ImplicitActuatorCfg(
            joint_names_expr=["drive_joint"],
            stiffness=None,#0.54539,
            damping=None,#0.00022,
        ),