Forward Kinematics

Kinematic Chains

• Simply collection of links and joints

• Different values of the joints mean that the robot is moving in space

• Transforms associated with links never change, whereas, Transforms associated with joints change with the joint values.

Definition of Forward Kinematics: The Task of computing the pose of all links (including end effector) given the joint angles.

URDF vs DH Notation

• URDF is used in ROS and research, whereas, DH is used in industries.

DH Notation

• meant to be compact way to describe robot

• Joint Axis is always Local Z. (that is, Axis for J(i) is Z(i-1) )

• L(i) can be translation along X(i) or rotation around X(i)

• Uses 4 numbers: theta, d, a, alpha

• Rotation around Z, Translation along Z, Translation along X, Rotation X

Goal of DH:

• A nice analytical formulation for the forward kinematics for a particular robot.

DH Notation Example 1 - 2 Link Planar Robot

DH Notation Example 2 - SCARA Robot

Rules

• If the transform has TRANSLATION first and then ROTATION, then you can concatenate the transform.

DH Notation Example 3 - 6 DOF and 7 DOF Robots

• All revolute joints and no prismatic joints

• Very common in Industrial Robots

• Simulation of 6 DOF Visualizer - GraspIT

Question: Why is 6 DOF a very common configuration?

Coding Challenge Forward Kinematics (by Daniel Shiffman)