Beschreibung
In this work, three different modules are designed to build compliant hyper-redundant mechanisms, used as backbone of robotic catheters. The current technology and approach includes co-centric, pre-shaped tubular structures or cable-driven piece-wise controlled catheters. Although both of these approaches are successful, there is still room for improvement if a novel approach including the compliant hyper-redundant mechanisms is taken. Unlike the tubular co-centric or cable-driven continuum/p.w. continuum structures, segment-based precise control of the structure can be achieved in both shape and stiffness control at local or global scale. In this study, we compare 3 modules based on their kinematics and variable-stiffness capability. Finally, we assess the controllability of the modules based on their structure and identify the trade-offs between shape-control and stiffness-control in navigations tasks, for example torturous vein passages especially in cardio-vascular operations. Preferred type of presentation: Oral I. MOTIVATION AND PROBLEM DEFINITION The main idea of a hyper-redundant robotic platform with a modular building block is to increase the controllability and maneuverability of the robotic catheters. Increasing the number of DoF seems to be the main advantage, however it is surpassed by the fully continuous robot (i.e. tubular/telescopic pre-curved continuum robots) that can be manipulated in 3D space without the need of lengthy inverse-kinematic calculations. Although fully-continuous robotic platforms have this advantage, for most cases, segment-based local-control is very difficult to obtain and despite the inherent compliancy, the stiffness control for most continuous robots is not possible. Therefore in this work, we propose three different modular hyper-redundant robotic designs that can offer segment-based position control as well as adjustable stiffness. When the robotic catheter has both the position and the stiffness control, the navigation of the robotic catheter inside torturous channels becomes an optimal control problem, where the position and force are controlled with varying priorities according to the path-planning and task. This greatly increases the safety of the robotic catheter. In this work, novel hyper-redundant modules are introduced and compared using kinematics and stiffness analysis. II. RELATED WORK One of the first robots helping surgeons in OR and performing some part of the operation is the DaVinci system, which has been already commercialized[1]. However, there is still need to develop versatile minimally invasive surgical robots with which the operations involving difficult-to-reach places can be safely performed. These types of surgical robots usually have a form of continuum or hyper-redundant structure and are remotely driven to navigate inside narrow and torturous channels[2]. The recently proposed hyper-redundant modular structures still use rigid or semi-rigid backbones or general frames[3],[4], [5]. The cable driven structures are lightweight and compact, however have a limitation due to cable friction and inter-dependency between the sections. Fig. 1. Three modules and their kinematic structures. III. OWN APPROACH AND CONTRIBUTION In this work, a systematical approach is taken to improve the design of hyper-redundant and modular robotic structures by emphasizing the functional properties such as independent module/segment control, variable-adjustable stiffness. The proposed designs are aimed at improving both position and force controllability. For this purpose, three modular structures are designed with at least three DoF each, allowing lightweight actuators to be embedded for segment control. In addition, variable stiffness at least for one axis is assured.The first one is a hybrid module (HM) with two parallel plates supported by a middle shaft featuring a universal-joint to obtain pan-tilt movement and having a compression spring around.The second module is inspired by nature and the previous studies on sea-horse-tail[6]. This module has a passive spherical joint in the middle and overlapping corner parts. The SHT has lateral and oblique muscles moving the exoskeleton of the seahorse. Using a similar approach, the SHT-module can contract and expand radially while the individual segments are able to perform rotations in 3D. Although the mechanism has no structural compliancy axially, it can have a great radial compliancy. Finally, the third module is a PKM with 3x SPS struts that can be reconfigured in a curvilinear manner.The module benefits from the compact and stable PKM while allowing the stiffness to be adjusted. The actuation units are located in prismatic joints and curvilinear adjustments. In terms of con-trollability, we find SHT and 3xSPS mechanisms more controllable due to the over-actuated structure and reduced uncertainty with the exclusion of the spring.
