# Review: Homogeneous Transformations

## Presentation on theme: "Review: Homogeneous Transformations"— Presentation transcript:

Review: Homogeneous Transformations
Homogeneous vector Homogeneous transformation matrix

Review: Aim of Direct Kinematics
Compute the position and orientation of the end effector as a function of the joint variables

Review: Direct Kinematics
The direct kinematics function is expressed by the homogeneous transformation matrix

Review: Open Chain Computation of direct kinematics function is recursive and systematic

Review: Denavit-Hartenberg Convention

Review : D-H Convention

Class Problem: Spherical Wrist
1. Fill in the table of D-H parameters for the spherical wrist. 2. write the three D-H transformation matrices (one for each joint) for the spherical wrist 3. Find the overall transformation matrix which relates the final coordinates (x6y6z6) to the “base” coordinates (x3y3z3) for the spherical wrist

Review : D-H Convention

Joint Space and Operational Space
Description of end-effector task position: coordinates (easy) orientation: (n s a) (difficult) w.r.t base frame Function of time Operational space Joint space Prismatic: d Revolute: theta Independent variables

Joint Space and Operational Space
Direct kinematics equation Three-link planar arm (Pp ) M的最大值是？平面运动只需要3个自由度

Joint Space and Operational Space
Generally not easy to express

Joint Space and Operational Space
Workspace reachable workspace dexterous workspace Factors determining workspace Manipulator geometry Mechanical joint limits Mathematical description of workspace Workspace is finite, closed, connected

Workspace Example

Performance Indexes of Manipulator
Accuracy of manipulator Deviation between the reached position and the position computed via direct kinematics. repeatability of manipulator A measure of the ability to return to a previously reached position.

Kinematic Redundancy Definition
A manipulator is termed kinematically redundant when it has a number of degrees of mobility which is greater than the number of variables that are necessary to describe a given task.

Kinematic Redundancy Intrinsic redundancy m<n functional redundancy
relative to the task Why to intentionally utilize redundancy?

Kinematic Calibration
Kinematic calibration techniques are devoted to finding accurate estimates of D-H parameters from a series of measurements on the manipulator’s end-effector location. Direct measurement of D-H is not allowed.

Inverse Kinematics

Inverse Kinematics we know the desired “world” or “base” coordinates for the end-effector or tool we need to compute the set of joint coordinates that will give us this desired position (and orientation in the 6-link case). the inverse kinematics problem is much more difficult than the forward problem!

Inverse Kinematics there is no general purpose technique that will guarantee a closed-form solution to the inverse problem! Multiple solutions may exist Infinite solutions may exist, e.g., in the case of redundancy There might be no admissible solutions (condition: x in (dexterous) workspace)

Inverse Kinematics most solution techniques (particularly the one shown below) rely a great deal on geometric or algebraic insight and a few common “tricks” to generate a closed-form solution Numerical solution techniques may be applied to all problems, but in general do not allow computation of all admissible solutions

Three-link Planar Arm x is known, compute q

Three-link Planar Arm W can be expressed both as a function of end-effector p&o, and as a function of a reduced number of joint variables

Three-link Planar Arm no admissible solution If c2 is out of this range Elbow up and elbow down