Wen Bin Lim

Optimization of Tension Distribution for Cable-Driven Manipulators Using Tension-Level Index

Cable-driven manipulators (CDMs) are a special class of parallel manipulators driven by cables instead of rigid links. Due to the unilateral driving property of cables, CDMs require redundant actuation to maintain positive cable tensions. As a result of actuation redundancy, there exist an infinite number of possible cable tension solutions for a particular CDM pose. In this paper, a tension optimization method is proposed to obtain adjustable tension solution for CDMs. Adjustable tension solution is important because it enables the avoidance of tension limits and allows the stiffness of the CDM to be regulated. Tension-level indices are introduced to provide a systematic and practical way of setting the desired cable tensions level. The adjustable tension solution for each pose is obtained using a modified gradient projection method. The proposed method is generic and can be applied to CDMs with any number of redundant actuation. Simulation results show that the method is computationally efficient and the tension solutions can be manipulated by changing the tension-level indices. The simulation results have also been validated by experimental studies.

A generic tension-closure analysis method for fully-constrained cable-driven parallel manipulators

Cable-driven parallel manipulators (CDPMs) are a special class of parallel manipulators that are driven by cables instead of rigid links. Due to the unilateral property of the cables, all the driving cables in a fully-constrained CDPM must always maintain positive tension. As a result, tension analysis is the most essential issue for these CDPMs. By drawing upon the mathematical theory from convex analysis, a sufficient and necessary tension-closure condition is proposed in this paper. The key point of this tension-closure condition is to construct a critical vector that must be positively expressed by the tension vectors associated with the driving cables. It has been verified that such a tension-closure condition is general enough to cater for CDPMs with different numbers of cables and DOFs. Using the tension-closure condition, a computationally efficient algorithm is developed for the tension-closure pose analysis of CDPMs, in which only a limited set of deterministic linear equation systems need to be resolved. This algorithm has been employed for the tension-closure workspace analysis of CDPMs and verified by a number of computational examples. The computational time required by the proposed algorithm is always shorter as compared to other existing algorithms.

Kinematic analysis and design optimization of a cable-driven universal joint module

Cable-driven parallel manipulators (CDPMs) are a special class of parallel manipulators that are driven by cables instead of rigid links. So CDPMs have a light-weight structure with large reachable workspace. The aim of this paper is to provide the kinematic analysis and the design optimization of a cable-driven 2-DOF module, comprised of a passive universal joint, for a reconfigurable system. This universal joint module can be part of a modular reconfigurable system where various cable-driven modules can be attached serially into many different configurations. Based on a symmetric design approach, six topological configurations are enumerated with three or four cables arrangements. With a variable constrained axis, the structure matrix of the universal joint has to be formulated with respect to the intermediate frame. The orientation workspace of the universal joint is a submanifold of SO(3). Therefore, the workspace representation is a plane in R2. With the integral measure for the submanifold expressed as a cosine function of one of the angles of rotation, an equivolumetric method is employed to numerically calculate the workspace volume. The orientation workspace volume of the universal joint module is found to be 2π. Optimization results show that the 4-1 cable arrangement produces the largest workspace with better Global Conditioning Index.

Design and analysis of a cable-driven manipulator with variable stiffness

A manipulator with variable stiffness allows the manipulator to adjust its stiffness to fulfill different task requirements. In this paper, a cable-driven manipulator with the ability to significantly regulate its stiffness through tension manipulation is introduced. Variable stiffness is achieved by attaching a novel variable stiffness device along each driving cable, in which the stiffness of the device is a function the cable tension. As cable-driven manipulator has actuation redundancy, the tension distribution can be manipulated even at a stationary pose. Such property allows the cable-driven manipulator to adjust the stiffness of each variable stiffness device, thereby changing the stiffness of the manipulator. The design and analysis of the variable stiffness device is presented. The variable stiffness device uses commercial torsion springs, and has a compact and light-weight design. Experimental and simulation results verified that cable-driven manipulator with such variable stiffness devices is able to achieve significant stiffness regulation.

Tension optimization for cable-driven parallel manipulators using gradient projection

Cable-driven parallel manipulators (CDPMs) are a special class of parallel manipulators that are driven by cables instead of rigid links. Cables have unilateral driving properties so that redundant actuation is required to maintain positive cable tensions. Due to the actuation redundancy, there exists an infinite number of tension solutions for every CDPMs pose. In this paper, a gradient projection method is employed for tension optimization of CDPMs. This optimization process minimizes a performance criterion which is a function of the cable tensions and the tension limits. The steepest descent direction of the performance criterion is projected onto the null space of the structure matrix to provide the direction towards the local minimum. Global optimal tension solution is found by repeating this process. A factor is introduced to adjust the tension solution towards the upper or lower tension limits. Simulation results show that the algorithm produces tension solutions that are continuous and smooth.

Design and motion control of a cable-driven dexterous robotic arm

Robots with hyper degrees of freedom motion are highly flexible, hence they are the ideal candidates to work in highly-confined spaces. These dexterous robotic arms are classified into two categories, i.e., discrete and continuum. As the weight of the discrete robots is a major concern, this paper discusses about the possibility of reducing their weight by replacing the heavy actuators with light cables. The proposed cable-driven robot consists of a series of identical parallel-structured modules connected to one another via universal joints. Experimental results showed that the cables have demonstrated a good step response with no steady-state errors and a fast settling time of 0.042 seconds. By using the kinematics formulations analysed in this paper, position control has been successfully implemented on a two-module cable-driven prototype. This allows the robot to be controlled in a co-ordinated manner while maintaining a smooth trajectory.

Design optimization of a cable-driven two-DOF joint module with a flexible backbone

A cable-driven snake-like robot arm (CDSLRA) with a flexible backbone possesses a number of promising advantages over the conventional rigid-link manipulators, such as lightweight mechanical structure, large reachable workspace, and high maneuverability. In general, a CDSLRA is a hyper-redundant manipulator that consists of a large number of active segments. As the basic building block of a CDSLRA, a 2-DOF cable-driven joint module with a flexible backbone is proposed. This paper focuses on the kinematics, kinetostatics and design optimization of the 2-DOF flexible joint module. Based on the 2-DOF joint module design as well as its resultant motion characteristics, the concept of instantaneous screw axis is proposed to formulate the kinematic model of the 2-DOF joint module, in which the Product-Of-Exponentials (POE) formula is employed. In order to generate the feasible workspace subject to the positive tension constraint, the kinetostatics of the 2-DOF cable-driven joint module is addressed, where the stiffness resulting from both the driving cables and the flexible backbone are considered. A numerical orientation workspace evaluation method is proposed based on an equi-volumetric partition in its parametric space and the volume-element associated integral factor. A multi-performance index, which takes both the stiffness condition index and the workspace volume into account, is employed to optimize the geometric size of the joint module. The simulation results demonstrate the effectiveness of the proposed multi-performance optimization algorithm.