Possible to add multiple actuators for one joint?

[Nate711]

I'd like to model a parallel elastic actuator by adding two ImplicitActuators to a single joint. One actuator would be controller normally (the motor), and the other actuator would have zero damping and have a set point of 0.0 to model the spring. Is this possible? Thanks!

I tried just adding another set of actuators to my ArticulationCfg like so
actuators={"base_legs": BASE_LEGS_CFG, "springs": SPRING_CFG},

but I got the following error
2025-04-03 20:12:23 [13,774ms] [Warning] [isaaclab.assets.articulation.articulation] Not all actuators are configured! Total number of actuated joints not equal to number of joints available: 28 != 16.

[RandomOakForest]

Thanks for posting this. There's definitely something missing. Could you share more on the configuration files? Also, there is a section in our docs on actuator groups that points to some examples. Have these been of any help? Thanks.

[Nate711]

Thanks for the reply! I didn't see any examples of multiple actuators attached to a single joint in the page you linked? Could you be more specific in which parts of the documentation to refer to?

Here is the articulation config I'm using

# TODO: Getting this error when trying to add multiple actuators to joints
# "springs": SPRING_CFG},
# 2025-04-03 20:12:23 [13,774ms] [Warning] [isaaclab.assets.articulation.articulation] Not all actuators are configured! Total number of actuated joints not equal to number of joints available: 28 != 16.
SPRING_CFG = ImplicitActuatorCfg(
    joint_names_expr=[".*abad", ".*hip", ".*knee"],
    effort_limit_sim=44,  # [N-m]p
    velocity_limit_sim=33.0,  # [rad/s]
    stiffness=75.0,  # [N-m/rad]
    damping=2.0,  # [N-m-s/rad]
    friction=0.0,  # [N-m-s/rad]
)

BASE_LEGS_CFG = ImplicitActuatorCfg(
    joint_names_expr=[".*abad", ".*hip", ".*knee"],
    effort_limit_sim=22,  # [N-m]
    velocity_limit_sim=33.0,  # [rad/s]
    stiffness=75.0,  # [N-m/rad]
    damping=2.0,  # [N-m-s/rad]
    friction=0.0,  # [N-m-s/rad]
)

INITIAL_BODY_POSITION = (0.0, 0.0, 0.53)
"""Initial position of the body in simulation world frame."""

ARTICULATION_PROPS = sim_utils.ArticulationRootPropertiesCfg(
    enabled_self_collisions=True,
    solver_position_iteration_count=4,
    solver_velocity_iteration_count=1,
)

ROBOT_CFG = ArticulationCfg(
    spawn=sim_utils.UsdFileCfg(
        usd_path=[my usd],
        activate_contact_sensors=True,
        rigid_props=sim_utils.RigidBodyPropertiesCfg(
            disable_gravity=False,
            retain_accelerations=False,
            linear_damping=0.0,
            angular_damping=0.0,
            max_linear_velocity=1000.0,
            max_angular_velocity=3000.0, 
            max_depenetration_velocity=1.0,
        ),
        articulation_props=ARTICULATION_PROPS,
    ),
    init_state=ArticulationCfg.InitialStateCfg(
        pos=INITIAL_BODY_POSITION,
        joint_pos={
            ".*abad": 0.0,
            ".*hip": 0.0,
            ".*knee": 0,
        },
        joint_vel={".*": 0.0},
    ),
    soft_joint_pos_limit_factor=0.9,
    actuators={"base_legs": BASE_LEGS_CFG,  "springs": SPRING_CFG},
)

[Nate711]

Should I just change this from an equality check to a < check?

IsaacLab/source/isaaclab/isaaclab/assets/articulation/articulation.py

Line 1388 in 7d3c7e5

if total_act_joints != (self.num_joints - self.num_fixed_tendons):

[Nate711]

@RandomOakForest