2.1. Kinematic model

A kinematic model describes the motion of a robot in mathematical form without considering the forces that affect motion and concerns itself with the geometric relationship between elements.

In the kinematic model of a robot, the connection of different manipulator joints is known as link, and the integration of two or more links is called a joint. This kinematic model can be represented as a tree structure. The tree describes the kinematic chain, i.e., the connection of robotic links with joints, and the inter-dependendencies of these links. This tree structure plus the underlying geometric information can be defined in Unified Robot Description Format (URDF), which describes any robot (see for example UR5 URDF). If the robot is mounted on external axes, these links and joints can be added as well.

../../_images/01_robot_links_and_joints.jpg

An industrial robot’s links and joints, and the according tree structure describing the kinematic model.

2.1.2. Joints

The joints are the elements in a robot which helps the links to travel in different kind of movements. The three major types of joints are:

  • Revolute: A hinge joint that rotates along the axis and has a limited range specified by the upper and lower limits.

  • Prismatic: A sliding joint that slides along the axis, and has a limited range specified by the upper and lower limits.

  • Fixed: Not really a joint because it cannot move, all degrees of freedom are locked.

COMPAS Framework provides classes to describe robot models. The following snippet shows how to describe a UR5 robot:

from compas.robots import RobotModel
from compas.robots import Joint
from compas.robots import Link

robot = RobotModel('ur5',
              joints=[
                  Joint('shoulder_pan_joint', 'revolute', parent='base_link', child='shoulder_link'),
                  Joint('shoulder_lift_joint', 'revolute', parent='shoulder_link', child='upper_arm_link'),
                  Joint('elbow_joint', 'revolute', parent='upper_arm_link', child='forearm_link'),
                  Joint('wrist_1_joint', 'revolute', parent='forearm_link', child='wrist_1_link'),
                  Joint('wrist_2_joint', 'revolute', parent='wrist_1_link', child='wrist_2_link'),
                  Joint('wrist_3_joint', 'revolute', parent='wrist_2_link', child='wrist_3_link'),
              ], links=[
                  Link('base_link'),
                  Link('shoulder_link'),
                  Link('upper_arm_link'),
                  Link('forearm_link'),
                  Link('wrist_1_link'),
                  Link('wrist_2_link'),
                  Link('wrist_3_link'),
              ])
print(robot)
../../_images/01_robot_model.jpg

Coordinate frames in each joint of the robot.