I kept it quite simple. I used set driven keys. I removed the parent relationships you had and parented each panel to the joint around which each panel should rotate. Added an attribute on the control cube and then used this attribute to drive the x rotation of each joint.

Also, have a look at chapter 4 of the rigging part of the "surfing with an Alien". It uses set driven keys to rig the fingers.

Your file is a bit dodgy. I got some errors opening it and for some or other reason I could not select the cube at the first joint so I just created a temporary one in the same place.