A robotic arm for an instrument having a rail, said robotic arm having a platform for coupling to said rail for movement of the robotic arm along a first axis, a gripper assembly, a first means, mounted to said platform and coupled to said gripper assembly, for moving said gripper assembly along a second axis perpendicular to said first axis, a second means, coupled to said platform, for pivoting said gripper assembly about said second axis, and an absolute encoder coupled to said gripper assembly, wherein said absolute encoder comprises a disk, having a center, a first sector and second sector, said second sector having a larger outer diameter than said first sector, and a sensor positioned adjacent said disk to detect said larger outer diameter of said second sector of said disk but not said first sector.