What you need is an IK controller directly connected to "Arm 2", because you actually don't want the "Point down" bone to be controlled by the IK - it should only be connected to "Arm 2" if I understand correctly:

Then the "Point down" bone is still connected to "Arm 2" but not part of the IK chain. Now it only needs a Limit Rotation constraint set to limit the X rotation in World Space to Min/Max -90°:

And now the "IK controller" bone controls the movement of the robot arm, the "Point down" moves with "Arm 2" but is always pointing down while having the same Y rotation as the "Base" bone:

What you need is an IK controller directly connected to "Arm 2", because you actually don't want the "Point down" bone to be controlled by the IK - it should only be connected to "Arm 2" if I understand correctly:

Then the "Point down" bone is still connected to "Arm 2" but not part of the IK chain. Now it only needs a Limit Rotation constraint set to limit the X rotation in World Space to Min/Max -90° (Local with Parent works too I think but then you need to set the limits to 0°):

And now the "IK controller" bone controls the movement of the robot arm, the "Point down" moves with "Arm 2" but is always pointing down while having the same Y rotation as the "Base" bone: