Many low-cost hobby class robots have only 4 joints (degrees of freedom). This document describes how to determine inverse kinematics for such a robot using the Robotics Toolbox for MATLAB.

Underactuation complicates the process of finding an inverse kinematic solution, and it frustrates those who are new to robotics — those who just want to run the code and get an answer. For a robot with 6 joints it’s quite straightforward, but underactuation requires some careful thought about the problem that you are trying to solve — you can’t just blindly use the tools.

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4 is harder than 6: Inverse kinematics for underactuated robots