URDF and SRDF¶

URDF Resources¶

• URDF ROS Wiki Page - The URDF ROS Wiki page is the source of most information about the URDF.
• URDF Tutorials - Tutorials for working with the URDF.
• SOLIDWORKS URDF Plugin - A plugin that lets you generate a URDF directly from a SOLIDWORKS model.

Important Tips¶

This section contains a set of tips on making sure that the URDF that you generate can be used with MoveIt. Make sure you go through all these tips before starting to use MoveIt with your robot.

<safety_controller k_position="100" k_velocity="1.5" soft_lower_limit="-2.857" soft_upper_limit="2.857"/>

URDF Examples¶

There are lots of URDFs available for robots using ROS.

• URDF Examples - A list of URDFs from the ROS community.

Virtual Joints¶

The URDF contains information only about the physical joints on the robot. Often, additional joints need to be defined to specify the pose of the root link on the robot with respect to a world coordinate system. In such cases, a virtual joint is used to specify this connection. E.g., a mobile robot like the PR2 that moves around in the plane is specified using a planar virtual joint that attaches the world coordinate frame to the frame of the robot. A fixed robot (like an industrial manipulator) should be attached to the world using a fixed joint.

Passive Joints¶

Passive joints are unactuated joints on a robot, e.g. passive casters on a differential drive robots. They are specified separately in the SRDF to make sure that different components in the motion planning or control pipelines know that the joints cannot be directly controlled. If your robot has unactuated casters, they should be specified as passive casters.

Groups¶

a ‘Group’ (sometimes called ‘JointGroup’ or ‘Planning Group’) is a central concept in MoveIt. MoveIt always acts on a particular group. MoveIt will only consider moving the joints in the group that it is planning for – other joints are left stationary. (A motion plan where all joints in the robot may move can be achieved by creating a group containing all joints.) A group is simply a collection of joints and links. Each group can be specified in one of several different ways:

End-Effectors¶

Certain groups in a robot can be given a special designation as an end-effector. An end-effector is typically connected to another group (like an arm) through a fixed joint. Note that when specifying groups that are end-effectors, it’s important to make sure that there are no common links between the end-effector and the parent group it is connected to.

Self-Collisions¶

The Default Self-Collision Matrix Generator (part of Setup Assistant) searches for pairs of links on the robot that can safely be disabled from collision checking, decreasing motion planning processing time. These pairs of links are disabled when they are always in collision, never in collision, in collision in the robot’s default position or when the links are adjacent to each other on the kinematic chain. The sampling density specifies how many random robot positions to check for self collision. Higher densities require more computation time while lower densities have a higher possibility of disabling pairs that should not be disabled. The default value is 10,000 collision checks. Collision checking is done in parallel to decrease processing time.

Robot Poses¶

The SRDF can also store fixed configurations of the robot. A typical example of the SRDF in this case is in defining a HOME position for a manipulator. The configuration is stored with a string id, which can be used to recover the configuration later.

SRDF Documentation¶

For information about the syntax for the SRDF, read more details on the ROS SRDF Wiki page.