# Joint Types And Limits > --- name: joint-types-and-limits description: 'Configure URDF joint types and limits. Use when setting up fixed, continuous, revolute, or prismatic joints with effort/velocity limits.' --- # Joint Types and Limits ## Joint Type Reference ### Fixed Joint No motion—rigidly attaches child to parent. Used for sensor mounts, structural elements, and `base_footprint → base_link`. ```xml ``` No `` or `` needed for fixed joints. ### Continuous Joint Unlimited rotation around a single axis. No position limits. Used for drive wheels and free-spinning casters. ```xml ``` Even continuous joints should specify `effort` and `velocity` limits for Gazebo enforcement. ### Revolute Joint Bounded rotation. Requires `` with `lower` and `upper` (in radians). ```xml ``` ### Prismatic Joint Linear sliding along an axis. Limits in meters. ```xml ``` ## Joint Sub-Elements ### `` Transform from parent link frame to the joint frame. Not the child link origin—the child link frame coincides with the joint frame at zero position. ```xml ``` ### `` The axis of rotation (revolute/continuous) or translation (prismatic). Normalized vector in the **joint frame**. ```xml ``` ### `` | Attribute | Unit | Required For | |---|---|---| | `lower` | rad / m | revolute, prismatic | | `upper` | rad / m | revolute, prismatic | | `effort` | N·m / N | all movable joints | | `velocity` | rad/s / m/s | all movable joints | ```xml ``` For continuous joints, `lower` and `upper` are ignored but `effort` and `velocity` are still enforced by Gazebo. ### `` ```xml ``` - **damping**: viscous damping coefficient (N·m·s/rad). Higher values slow rotation. - **friction**: Coulomb friction (N·m). Static resistance to motion. These affect simulation fidelity. Start with small values (0.01–0.1) and tune. ## Safety Controller Applies soft limits before the hard stops defined in ``: ```xml ``` The joint decelerates when entering the soft limit zone, preventing impact at the hard stop. Primarily used with `ros2_control` hardware interfaces. ## Mimic Joints For coupled joints where one joint's position is a function of another: ```xml ``` - **multiplier**: `mimic_position = multiplier * leader_position + offset` - **offset**: constant position offset - In Gazebo, mimic joints need a plugin (e.g., `gz-sim-joint-controller-system`) or a ros2_control mimic interface. ## Complete Pan-Tilt Example ```xml ``` ## Debugging Joint Issues ```bash # Verify joint limits in the parsed URDF check_urdf robot.urdf # In Gazebo: watch for "Joint position limit reached" warnings # In ros2_control: check /joint_states for clamped values # Inspect current joint states ros2 topic echo /joint_states ``` Common pitfalls: - Forgetting `effort`/`velocity` limits → Gazebo applies zero effort → joint won't move - Axis direction wrong → wheel spins sideways or joint rotates unexpectedly - Origin placed at child link center instead of at the joint pivot point