I've got a robot assembly arm (http://sendvid.com/ze9l813s) set up with an inverse kinematics solver, but I don't know how to keep the end bone from rotating. I need that bone to remain perpendicular to the Z axis, and to the target, while it is moving. I don't want to use rotation constraints, I just want the IK chain to have the end bone remain totally perpendicular to the ground. How do I do this with inverse kinematics?
If you're having trouble visualizing it, here's a video of what an assembly arm is supposed to work like: https://www.youtube.com/watch?v=J23oqwyN9-A
The arm can move straight up and straight down, not in some kind of weird bendy arc like the Inverse Kinematics rig is doing.