FBG-based shape sensing tubes for continuum robots

Abstract

Fiber Bragg gratings (FBG)-based optical sensors are a promising real-time technique for sensing the 3D curva- ture of continuum robots. Existing implementations, however, have relied on embedding optical fibers in small-diameter metal wires or needles. This paper proposes polymer tubes as an alternative substrate for the fibers. This approach separates the sensors from the robot structural components while using a minimal amount of the robot's tool lumen and providing the potential of inexpensive fabrication. Since the fibers are stiffer than the polymer substrate, however, design challenges arise in modeling strain transfer between the fibers and the tube substrate. To investigate the potential of this approach, a strain transfer model is derived and validated through simulation and experiment. I. INTRODUCTION Continuum robots take the shape of three dimensional curves and are able to change their shape through a com- bination of bending, rotation and extension or contraction of their structural components. Because of these capabilities, continuum robots are ideally suited for applications such as minimally invasive surgery. Their flexibility, however, leads to uncertainty in the shape of their backbone curve as well as in the location of their tip. Approaches to real-time sensing that have been studied include imaging, electromagnetic (EM) tracking and force sensing for tendon-based actuation. These techniques all have shortcomings, however. For example, drawbacks with imag- ing include limited resolution (ultrasound), risk of ionizing radiation (x-ray or CT), and slow speed (MRI). Tendon length-based shape estimation is limited to single bends and its accuracy depends highly on the robot kinematic model. In contrast to these approaches, a real-time sensing tech- nology that could be easily inserted and removed from the robot is preferred because of its direct measurement, instant adaptability to the continuous shape change and ease of replaceability. Curvature sensing using fiber Bragg gratings (FBG) has received recent attention, due to its small size, biocompatibility and high sensitivity. Compared to EM sensors (1), FBG-based sensors are smaller, immune to EM noise and can contain multiple sensors along the length of a fiber. For example designs using attached (2) or embedded (3), (4) fibers have been implemented on 1mm diameter metal wires. Both approaches reasonably assumed a perfect strain transfer from the wire to FBG. While the small wire diameter had minimal effect on structural stiffness and made it possible to accommodate large curvatures (gratings