Robots and worlds for simulation and motion planning

A robot is described for its consumers in XML: a URDF for its structure (ROS, RViz, robot_state_publisher), an SRDF beside it for MoveIt's planning semantics, and an MJCF for MuJoCo. There are two routes to one:

• Generate it from CAD. A text-to-cad model that holds the mechanism (build123d joints, mechanism states, couplings), materials and the frames it names is written as an MJCF or a URDF by cad model export, or made a mujoco.MjSpec by cad.mujoco.spec, on which a MuJoCo function makes a simulation's choices (collision shapes, joint dynamics, actuators). Frames, limits, masses, inertias and meshes come from the kernel, exactly: the model is the source of truth, and the file is regenerated, never edited.
• Author it by hand. A robot with no CAD model here, an SRDF (always), a MuJoCo scene, or a file that already exists: the .urdf, .srdf or MJCF .xml is the source of truth, written and edited directly. The work is constrained kinematic modeling, not XML writing: the risks are frame placement, joint-axis semantics, units, mesh scale, inertial data and planning semantics, and a file can pass every check while placing a joint in the wrong spot.

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