Robot Description

Robot Description#

The robot description contains semantic information about the robot which can not be extracted from the URDF in a general way. This includes kinematic chains, end-effectors, cameras and their parameter, etc.

In general a Robot Description consists a number of different descriptions, these are:

  • RobotDescription

  • KinematicChainDescription

  • EndEffectorDescription

  • CameraDescription

In this example we will create a robot description step-by-step and describe the different components on the way. The robot we will use as an example will be the PR2, the complete PR2 description can also be seen in pycram.robot_descriptions.pr2_description().

Robot Description Class#

We start by creating an instance of the RobotDescription class, this will serve as a the main component to which all other descriptions will be added.

To initialize a RobotDescription we need a few parameter which are:

  • Name

  • base_link

  • torso_link

  • torso_joint

  • Path to a URDF file

from pycram.robot_description import RobotDescription
from pycram.ros import get_ros_package_path

filename = get_ros_package_path('pycram') + '/resources/robots/' + "pr2" + '.urdf'
pr2_description = RobotDescription("pr2_example", "base_link", "torso_lift_link", "torso_lift_joint", filename)

Kinematic Chain Description#

The kinematic chain description describes a chain of links and joints of the robot which might be interesting when working with the robot. An example of such a chain would be the arm of the robot, when programming for the robot it is important to know which links and joints exactly make up the arm, however, these can not be extracted from the URDF automatically.

The kinematic chain is based upon the URDF, meaning when initializing the description one only needs to specify the first and last link of the chain.

We will now create the kinematic chain description for the right arm of the PR2. For initializing the KinematicChainDescription the following parameter are needed:

  • Name

  • first link

  • last link

  • URDF object

  • Arm type

The arm type specifies which arm this kinematic chain describes, this is needed when one wants to access only the arms of the robot.

from pycram.robot_description import KinematicChainDescription
from pycram.datastructures.enums import Arms

right_arm = KinematicChainDescription("right", "torso_lift_link", "r_wrist_roll_link",
                                      pr2_description.urdf_object, arm_type=Arms.RIGHT)

The created KinematicChainDescription can now be added to the robot description.

pr2_description.add_kinematic_chain_description(right_arm)

End Effector Description#

Since kinematic chains only describe a moveable chain of links and joints like arms these do not represent end-effectors which can be used to do manipulation tasks.

To represent end-effectors we will create an EndEffectorDescription which contains the information of the respective end-effector. When creating an EndEffectorDescription we need the following parameter:

  • Name

  • first link

  • tool_frame

  • URDF object

You might have noticed that the end-efftor only has a first link but no last link, this is the case since end-effectors are at the end of the arms. Therefore, all links and joints below a certain link can be seen as part of the end-effector.

from pycram.robot_description import EndEffectorDescription

right_gripper = EndEffectorDescription("right_gripper", "r_gripper_palm_link", "r_gripper_tool_frame",
                                       pr2_description.urdf_object)

The gripper can no be added to the previously created KinematicChainDescription.

right_arm.end_effector = right_gripper

Camera Description#

The camera description contains all parameters of a camera, which is mounted on the robot. The parameter for the CameraDescription are:

  • Name

  • Link name

  • minimal height

  • maximal height

  • horizontal angle

  • vertical angle

from pycram.robot_description import CameraDescription
from pycram.datastructures.enums import Grasp

camera = CameraDescription("kinect_camera", "wide_stereo_optical_frame", 1.27,
                           1.60, 0.99483, 0.75049)

The finished camera description can now be added to the robot description.

pr2_description.add_camera_description(camera)

Grasps#

Grasps define how a robot interacts with objects. The grasps defined in the robot description are set for each end-effector individually. The predefined grasp used is the Grasp.FRONT grasp of the robot. Based on this grasp, all other grasps are generated.

right_gripper.update_all_grasp_orientations([0, 0, 0, 1])

Register Robot Description#

Lastly, you need to register the robot description to the RobotDescriptionManager. As you can see the code to register the robot description has to be executed at the start of PyCRAM, if you put your file with the robot description in the pycram.robot_descriptions directory it will be executed upon the start of PyCRAM.

from pycram.robot_description import RobotDescriptionManager

rdm = RobotDescriptionManager()
rdm.register_description(pr2_description)
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