A positioner robot with two degrees of freedom. The main disk (black circle) rotates along its central axis. Its angular position is shown as θ_i. The arm with the length of l₂ rotates around an eccentric axis (with the distance of l₁ from the center) fixed on the main disk and its angular position is shown as φ_i. q_ib is the position of the robot base fixed to the telescope structure in a global frame attached to the focal plane. q_i is the position of the fiber attached to the robot i in that global frame.