Robot Description in PyCRAM

The robot description assigns meaning to the physical description of a robot from the URDF. This includes manipulator chains, meaning the links and joints which are part of an arm, the camera link and predefined poses for joint chains, like for example a default parking pose for an arm.

On this page we will list the parts of the robot description and explain them, for a tutorial on how to create your own robot description please look at the tutorial.

The robot_description file contains 6 classes:

• ChainDescription
• GripperDescription
• InteractionDescription
• ManipulatorDescription
• CameraDescriptin
• RobotDescription

This class represents a kinematic chain by saving the links and joints. This chain can for example be an arm of a robot. The parameter of the constructor along with a description and the type can be seen in the table below.

The class provides a number of methods to interact with static joint chains. These methods provide interfaces to add one or more joint chains or get a specific saved joint chain.

Adds a specific configuration of the joints saved in this chain. For example a static joint chain would be the default parking pose of an arm of a robot.

This method lets you add more than one static joint chain at a time. The different static joint chains are given in a dictionary with the name of the static joint chain as key and the list of states as value.

This method returns a static joint chain, given the name of the static joint chain.

This class inherits from the ChainDescrioption class and describes a gripper of a robot. This class allows you to specify more details about the gripper as well as specify joint configurations for this gripper. The constructor of this class takes a lot of additional parameter to describe the gripper however all except for the name are optional. The parameter of the constructor will be explained in the table below.

The GripperDescription class doesn't define any methods. However, since it inherits from the ChainDescription class all methods of the ChainDescription class are also available.

This class allows to put on the end of an chain another link, which is saved as an end effector. Therefore, chains can be defined which specify the interaction frame for the robot. An example could be a storage place for grasped objects on the robot. The parameter of the constructor can be seen in the table below.

      chain of links: chain_description <-> eef_link

This class doesn't define any methods but since it inherits from the ChainDescription class all methods of the ChainDescription are also available.

This class allows with the given interaction description to include a gripper description which is placed between the last link of the interaction description and the rest of it. Independently from that a tool frame can be saved, which allows to use objects to manipulate the environment.

                                                                         |--> (tool_frame)
      chain of links: interaction_description <-> (gripper_description) -|
                                                                         |--> eef_link
                                                                         

The parameter of the constructor will be shown in the following table.

This class doesn't define any methods but since it inherits from the InteractionDescription class all methods of the InteractionDescription are also available.

This class represents a camera of the robot, it saves the link in the URDF as well as additional parameter which describe physical properties of the camera. The parameter of the constructor of this class can be seen in the table below.

This class doesn't define any methods.

The RobotDescription is an abstract class which needs to be inherited from to implement a robot description for a specific robot. It implements different functions to add and get chains of different objects types which inherit of the class ChainDescription. Moreover, it allows to model the robot with its odom_frame, base_frame, base_link and torso links and joints. Different cameras can be added and static transforms and poses can be added too. The parameter of the constructor can be seen in the table below.

There are a number of methods defined by the RobotDescription class to add more information or retrieve saved informations about the robot. In the following we will take a close look at all methods and explain the parameter these methods take.

This method returns the saved tool frame of a manipulator description, saved by the given name.

Returns a static joint chain given the name of a ChainDescription and a configuration of this chain. The chain name is the name which identifies a particular chain description in the robot description. For example, 'left' for a chain description describing the left arm of a robot.

This method adds a chain description with a name identifying this chain. For example, the name is 'left' and the chain description describes the left arm of a robot.

This method lets you add more than one chain at a time by taking a dictionary composed of the name identifying the chain as key and the chain description as value.

This method adds a camera description to this robot description. The method uses, like add_chain, a name to identify the camera description.

This method lets you add more than one camera at a time by taking a dictionary containing multiple camera descriptions. The key of the dictionary functions as the name to identify the camera description in the corresponding value.

Returns the camera frame for a given name of a camera description.

This method adds a static joint chain to a chain description saved in this robot description. See also add_static_joint_chain of the ChainDescription above.

This method allows you to add multiple static joint chains at once. See also add_static_joint_chains of the ChainDescription above.

This method adds a static joint chain to a saved manipulator description. The static joint chain is meant to be used for the gripper.

This method adds more than one static joint chain to a manipulator description.

Returns a static gripper joint chain for a given manipulator description and configuration.