# Inverse kinematics for a 4 DOF arm

I am working with the openMANIPULATOR-X robotic arm and I am trying to implement inverse kinematics for it. I have found this article that talks about kinematic modelling:

https://pdfs.semanticscholar.org/56a0/9731772199fe77bcb0dbff3a69f1c623e0c7.pdf

But when I implemented it in matlab, theta_3 had a complex number indicating that it is not possible for the robotic arm which does not make sense. I understand how everything is derived in the paper but it seems I am missing something. Could anyone help me?

• when I implemented it in matlab ... did you get an error? ... please edit any error listing into the question ... do not use comments Feb 21 at 5:35
• Hi @Midnight 2 as jsotola mentioned please edit your question to provide enough information to reproduce your problem including your minimal self contained example of your error. We can only guess if you're missing something without seeing your implementation. Feb 23 at 18:22
• The algorithm in the paper describes how to model how the joint angles vary over time to support the desired end effector trajectory. $\theta_3$ is a time series. Did you plot how it varies over time to get some insight into what might be happenning? Mar 27 at 12:36