Design and analysis of a modular reconfigurable cable-driven manipulator system

• Main Author: Lim, Wenbin.
• Other Authors: Yeo Song Huat
• Format: Theses and Dissertations
• Language: English
• Published: 2013
• Subjects: DRNTU::Engineering::Mechanical engineering::Robots; DRNTU::Engineering::Mechanical engineering::Mechatronics
• Online Access: http://hdl.handle.net/10356/51167
• Institution: Nanyang Technological University

Modular manipulators are robotic systems that are built from many identical but independent modules. They have the advantages of flexibility, reconfigurability for rapid deployment and ease-of-maintenance. However, many existing modular manipulators are rigid-link arms with their actuators integrated on the modules, making them bulky, heavy, and having high moments of inertia. They are therefore not suitable for applications in confined space inspection and unsafe for use as assistive robots. The objective of this research is therefore to analyse and develop a modular cable-driven manipulator system that has the advantages of large reachable workspace, high payload-to-weight ratio, high flexibility, high dexterity, variable stiffness, and can produce intrinsically-safe motion. In this report, the kinematic analysis for the {\it{cable-driven universal joint}} (CDUJ) module and the {\it{cable-driven robotic arm}} (CDRA) are addressed. At the module level, the force-closure condition and the tension optimisation are investigated. The proposed force-closure algorithm, which satisfies the necessary and sufficient condition, is generic, easy-to-implement and computationally efficient. The design of the CDUJ module is optimised to achieve a large force-closure workspace and high payload-to-weight ratio. A gradient projection based tension optimisation method is proposed to obtain adjustable tension solution for each pose. Tension adjustment factors are introduced to manipulate the cable tensions so as to regulate the stiffness of the module. The stiffness of the CDUJ module is contributed by both the cable stiffness and the cable tensions. It is found that the cable tensions do not produce significant changes to the stiffness. Hence, a variable-stiffness device (VSD) is proposed to be attached to the cables so that the stiffness of the module can be significantly regulated through tension manipulation. At a particular pose, the tension distribution required to meet the stiffness requirement can be determined using a stiffness-oriented tension resolution algorithm. Adjustable stiffness allows the module to change its stiffness to suit different task requirements. At the manipulator level, a configuration-independent modeling approach based on the Product-of-Exponentials (POE) formula is employed to solve the kinematics of the CDRA. For a CDRA with redundant degrees of freedom, the weighed-least-norm inverse kinematics solution ensures that the modules operate within their workspace. Prototypes of CDUJ module are built and experiments are performed to verify the tension and stiffness analysis. Tension control is performed on the CDUJ modules using the joint-based hybrid control scheme. In addition, a CDRA prototype is also assembled using four CDUJ modules. It is 0.5m in length, weighs about 0.9kg, and can carry a load of 4kg. When the variable-stiffness devices are employed, experimental results showed that the stiffness of a module and the CDRA can be significantly regulated by manipulating the cable tensions.