I. Introduction
Recently, there has been intensive research on the concentric-tube continuum robot, so called active cannulas or steerable needle for minimally invasive surgery (MIS) [1] ~[7]. The most distinctive characteristic of this type of robot is that its distal end is steered and advances through rotation and translation of the pre-curved tubes relative to each other. Actuators are attached to the proximal ends of the tubes, and no other actuators are required such as pull-wires or pneumatic pumps [8], [9]. Therefore, the position of the robot's distal end and the robot's shape along it length can be controlled purely by rotating and translating the base of the tubes. This implicit actuating unit and the structural simplicity allow for small-scale design and give the robot great potential to be used as a new MIS instrument that incorporates the advantages of different types of conventional tools [10].