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Research Papers: Design of Mechanisms and Robotic Systems

Novel Fault-Tolerance Indices for Redundantly Actuated Parallel Robots

[+] Author and Article Information
Mats Isaksson

Electrical and Computer Engineering Department,
Fort Collins, CO 80523
e-mail: mats.isaksson@gmail.com

Kristan Marlow

Institute for Intelligent Systems
Research and Innovation,
Deakin University,
Waurn Ponds Campus,
Geelong, VIC 3217, Australia
e-mail: kristan.marlow@gmail.com

Anthony Maciejewski

Electrical and Computer Engineering Department,
Fort Collins, CO 80523
e-mail: aam@engr.colostate.edu

Anders Eriksson

School of Electrical Engineering and
Computer Science,
Queensland University of Technology,
GPO Box 2434,
Brisbane, QLD 4001 Australia
e-mail: anders.eriksson@qut.edu.au

Contributed by the Mechanisms and Robotics Committee of ASME for publication in the JOURNAL OF MECHANICAL DESIGN. Manuscript received July 13, 2016; final manuscript received December 15, 2016; published online January 31, 2017. Assoc. Editor: Oscar Altuzarra.

J. Mech. Des 139(4), 042301 (Jan 31, 2017) (10 pages) Paper No: MD-16-1504; doi: 10.1115/1.4035587 History: Received July 13, 2016; Revised December 15, 2016

Abstract

Robots designed for space applications, deep sea applications, handling of hazardous material and surgery should ideally be able to handle as many potential faults as possible. This paper provides novel indices for fault tolerance analysis of redundantly actuated parallel robots. Such robots have the potential for higher accuracy, improved stiffness, and higher acceleration compared to similar-sized serial robots. The faults considered are free-swinging joint failures (FSJFs), defined as a software or hardware fault, preventing the administration of actuator torque on a joint. However, for a large range of robots, the proposed indices are applicable also to faults corresponding to the disappearance of a kinematic chain, for example, a breakage. Most existing fault tolerance indices provide a ratio between a robot's performance after the fault and the performance before the fault. In contrast, the indices proposed in this paper provide absolute measures of a robot's performance under the worst-case faults. The proposed indices are based on two recently introduced metrics for motion/force transmission analysis of parallel robots. Their main advantage is their applicability to parallel robots with arbitrary degrees-of–freedom (DOF), along with their intuitive geometric interpretation. The feasibility of the proposed indices is demonstrated through application on a redundantly actuated planar parallel mechanism.

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Figures

Fig. 1

A 3-DOF planar redundantly actuated 4-RPR mechanism

Fig. 2

A redundantly actuated 3-DOF planar parallel mechanism with five actuators. (a) Top view of the mechanism and (b) side view, demonstrating how the mechanism can be designed in order to avoid collisions between the lower arm links and the manipulated platform when it is rotated.

Fig. 3

Calculation of ITI (a) and OTI (b) for one chain of a nonredundant submechanism obtained by removing two kinematic chains from the mechanism in Fig. 2(a). (a) ρIi is the length of the common perpendicular between the screws $̂A,i and$̂I,i while ρIimax is the maximum length of the common perpendicular obtained by rotating $̂A,i around ci and (b) ρOk is the length of the common perpendicular between the screws$̂A,k and $̂O,k while ρOkmax is the maximum length of the common perpendicular obtained by rotating$̂A,k around dk.

Fig. 4

Analysis of the mechanism in Fig. 2 for the same position (x=y=0) and all platform angles (ϕ). (a) The ITI value drawn in bold is the minimum of the ITI values for each of the five chains, which are drawn using narrow lines, (b) The LMTI of the redundantly actuated mechanism in Fig. 2 is drawn in bold while the narrow lines show the OTI of the ten nonredundant submechanisms required to calculate the LMTI, (c) The LMTI of the redundantly actuated submechanism not including link 5 is drawn in bold while the narrow lines show the OTI of the four nonredundant submechanisms required to calculate the LMTI, and (d) The output fault tolerance index FO1 is drawn in bold while the narrow lines show the LMTI for the five possible redundantly actuated mechanisms that can be obtained by removing one chain from the mechanism in Fig. 2(a).

Fig. 5

The positional workspace of the mechanism in Fig. 2. Each position is marked according to the lowest value of the studied index during a 360 deg platform rotation. (a) The ITI value, (b) The LMTI512345 value, (c) The output fault tolerance index FO1, and (d) The color map used for all plots. See figure online for color.

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