How to maintain IK chain angle to target? 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.
If I am understanding your question correctly, I think this kind of setup would be the simplest solution:



The main point is to disable "Inherit Rotation" for the tip bone.



Extrude another bone from the root of the "Tip Bone" that will be the "IK Target", and clear its Parent (AltP in Armature Edit Mode). It must not have a Parent in order to function properly as an IK Target.