Design and analysis of cable-driven snake-like robot arm with flexible backbone
Date of Issue2015
School of Mechanical and Aerospace Engineering
Robotics Research Centre
This thesis addresses the design and analysis of the cable-driven flexible snake-like robots (CFSR). CFSR have a number of potential advantages over the conventional rigid-link manipulators, such as lightweight mechanical structure, large reachable workspace, high dexterity, and high maneuverability, thus making them especially suitable for confined spaces applications. The aim of this thesis is to provide a frame work to effectively estimate the deformation and actuation force of the cable-driven flexible snake-like robot. To realize such estimations, two models are investigated, namely the simplification model and the general model. The modeling approaches proposed in this thesis can also be applied to other types of snake-like robots. The simplification model of a modular cable-driven flexible snake-like robot, consisting of a number of serially connected identical 2-DOF cable-driven joint module with a flexible backbone, is first presented. This simplification model is based on the constant curvature approximation, thus enabling the closed-form kinematics and closed-form Jacobian formulation. In particular, the instantaneous screw axis concept is proposed in this thesis such that the 2-DOF bending motion of the joint module can be concisely described by one rotation about an instantaneous screw axis. With such a geometrically meaningful concept, the Product-Of-Exponentials (POE) formula approaches then employed to formulate the kinematic model for the 2-DOF joint module, making the analysis significantly simplified. The instantaneous kinematics analysis based on lie group theory and numerical inverse kinematics (NIK) algorithm is also formulated for the purpose of the trajectory tracking and motion control. To obtain an optimal design, a global singular value index (GSV), which considers the minimum singular value of the stiffness matrix of the joint module over the achievable workspace, is proposed. The simplification model does not characterize the external forces and moments applied to the robot such as cable tension, or gravitational loading. The general model based on the nonlinear elasticity analysis is therefore introduced. It can provide more accurate estimations of the configurations and actuation force by considering real-world effects, such as gravitational loading or cable tension. In addition, since the general model is described by a nonlinear ordinary differential equation (ODE) system which needs good initial guesses for the solution. Here, the simplification model can provide effective initial guesses to solve the general model. The general model is used to investigate the relationship between the exerted force and configuration of the robot. The mechanical analysis of the backbone is based on the well-known Cosserat rod theory, because the configuration of the robot may cause large-deflection deformation, resulting in the material and geometric nonlinearities. For the general model, the forward problem and inverse problem in mechanics of the cable-driven flexible snake-like robot are both investigated. The forward problem is to determine the spatial deformation of the robot induced by known tension forces. The inverse problem is to investigate the tension forces from the given tip position. The results indicate that the general model provides an efficient and accurate approach to estimate the configuration and actuation forces so as to enable the research on motion planning and force control in the future work. Conclusion and future work are also included.