Research Papers

A Novel Joint Design for Robotic Hands With Humanlike Nonlinear Compliance

[+] Author and Article Information
Pei-Hsin Kuo

ReNeu Robotics Laboratory,
Mechanical Engineering,
The University of Texas,
Austin, TX 78712
e-mail: peihsin.kuo@utexas.edu

Ashish D. Deshpande

ReNeu Robotics Laboratory,
Mechanical Engineering,
The University of Texas,
Austin, TX 78712
e-mail: ashish@austin.utexas.edu

1Corresponding author.

Manuscript received December 6, 2014; final manuscript received July 31, 2015; published online November 24, 2015. Assoc. Editor: Marcia K. O'Malley.

J. Mechanisms Robotics 8(2), 021004 (Nov 24, 2015) (10 pages) Paper No: JMR-14-1335; doi: 10.1115/1.4031300 History: Received December 06, 2014; Revised July 31, 2015; Accepted August 08, 2015

Robotic hands are typically too rigid to react against unexpected impacts and disturbances in order to prevent damage. Human hands have great versatility and robustness due, in part, to the passive compliance at the hand joints. In this paper, we present a novel design for joint with passive compliance that is inspired by biomechanical properties of the human hands. The design consists of a compliant material and a set of pulleys that rotate and stretch the material as the joint rotates. We created six different compliant materials, and we optimized the joint design to match the desired humanlike compliance. We present two design features that allow for the tuning of the joint torque profile, namely, a pretension mechanism to increase pretension of the compliant material, and a design of varying pulley configuration. We built a prototype for the new joint by using additive manufacturing to fabricate the design components and built a test-bed with a force sensor and a servo motor. Experimental results show that the joint exhibits a nonlinear, double exponential joint compliance with all six compliant materials. The design feature involving variable pulley configurations is effective in adjusting the slope of joint torque during the joint rotation while the pretension mechanism showed only a limited effect on increasing the torque amplitude. Overall, with its small size, light weight, low friction, and humanlike joint compliance, the presented joint design is ready for implementation in robotic hands.

Copyright © 2016 by ASME
Topics: Torque , Design , Pulleys , Robotics
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Fig. 1

The underlying configuration for variable stiffness with the compliant materials at the initial position θd=0 deg (a), and at the joint angle θd=90 deg (b). The curved arrows show that the pulleys rotated about the center of the joint.

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Fig. 2

A schematic diagram of the rotated joint configuration and geometrical representations for 180 deg joint rotation

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Fig. 3

Results from optimization of design variables and parameters

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Fig. 4

A schematic diagram of off-center pulley and its new geometrical parameters due to the rotation in counterclockwise direction. The pulley is rotated about the off-center C with the angle θr, causing a change of geometrical engagement between the pulley and the material such that the new center of pulley (O′), radius (R′), and the distance R′ change the effective MA.

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Fig. 5

Five pulley configurations to modify the MA and hence the passive torque profile in the joint design

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Fig. 6

The components of parallel compliant joint design and the computer-aided design. Over 21 parts of the design are fabricated by a 3D printer.

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Fig. 7

A prototype of parallel compliant joint mechanism. Subsystems such as compliant material, variable pulley configuration, and pretension mechanism are shown in subfigures. The dimension of the joint mechanism is 25.46×23.18 mm (diameter × thickness) and weight is 5.34 g.

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Fig. 8

The stress–strain curves of six materials during the tensile test. The curves and their error bars represent the mean values and standard deviations of five specimens for each material.

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Fig. 9

The joint torque responses from the harmonic test with different cyclic frequencies for three S series applied materials

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Fig. 10

The experimental results of joint torque due to different pretensions on the compliant material. The embedded figures show that the pretension mechanism is moved to different positions.

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Fig. 11

Experimental joint torque results of tuning pulley angle and rotating direction. The basic configuration is the original pulley configuration without changing the angles.




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