Warn when free joint has non-NONE drive targets in SolverMuJoCo (#4083)

Author: aneangel

changelog/3701.fixed.md

@@ -0,0 +1 @@
+Warn when joint drive targets are set on a free joint under `SolverMuJoCo`. MuJoCo does not support actuators on free joints, so `joint_target_mode`, `joint_target_ke`, and `joint_target_kd` were silently ignored. A `UserWarning` is now emitted at solver construction time pointing at `Control.joint_f` as the supported workaround.

newton/_src/solvers/mujoco/solver_mujoco.py

@@ -6668,6 +6668,17 @@ def add_body_from_joint(j: int, *, mocap: bool | None):
                 num_dofs += 6
                 num_qpos += 7
                 num_mjc_joints += 1
+                for i in range(6):
+                    if joint_target_mode[qd_start + i] != int(JointTargetMode.NONE):
+                        warnings.warn(
+                            f"Free joint '{model.joint_label[j]}' has a non-NONE joint_target_mode but "
+                            "SolverMuJoCo cannot create actuators on free joints. "
+                            "Drive targets (joint_target_ke, joint_target_kd, joint_target_q) are silently ignored. "
+                            "Apply the desired wrench directly via Control.joint_f instead.",
+                            UserWarning,
+                            stacklevel=2,
+                        )
+                        break
             elif j_type == JointType.BALL:
                 ball_params = {
                     "name": name,