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RESEARCH PAPERS

The Design of a Chain of Spherical Stephenson Mechanisms for a Gearless Robotic Pitch-Roll Wrist

[+] Author and Article Information
S. Hernandez

Department of Mechanical Engineering & Centre for Intelligent Machines,  McGill University, Montreal, QC H3A 2A7 Canada

S. Bai

Department of Mechanical Engineering & Centre for Intelligent Machines,  McGill University, Montreal, QC H3A 2A7 Canadamspbai@cim.mcgill.ca

J. Angeles

Department of Mechanical Engineering & Centre for Intelligent Machines,  McGill University, Montreal, QC H3A 2A7 Canadaangeles@cim.mcgill.ca

J. Mech. Des 128(2), 422-429 (Jun 06, 2005) (8 pages) doi:10.1115/1.2167653 History: Received April 26, 2005; Revised June 06, 2005

Although bevel-gear robotic wrists are widely used in industrial manipulators due to their simple kinematics and low manufacturing cost, their gear trains function under rolling and sliding, the latter bringing about noise and vibration. Sliding is inherent to the straight teeth of the bevel gears of these trains. Moreover, unavoidable backlash introduces unmodeled dynamics, which mars robot performance. To alleviate these drawbacks, a gearless pitch-roll wrist is currently under development for low backlash and high stiffness. The wrist consists of spherical cam-rollers and spherical Stephenson linkages, besides two roller-carrying disks that drive a combination of cams and Stephenson mechanisms, the whole system rotating as a differential mechanism. The paper focuses on the design of the chain of spherical Stephenson mechanisms. The problem of the dimensional synthesis is addressed, and interference avoidance is discussed. An embodiment of the concept is also included.

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Copyright © 2006 by American Society of Mechanical Engineers
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Figures

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Figure 1

Gearless pitch-roll wrist based on cam-rollers: (1) outer cam; (2) roller-carrying disk; (3) gripper; (4) and (5) input shafts connected to the two motors; (6) roller; (7) inner cam

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Figure 2

A spherical Stephenson mechanism

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Figure 3

The four-bar loop

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Figure 4

The five-bar loop

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Figure 5

Arrangement of Stephenson mechanisms for rotation reversal

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Figure 6

Angular displacements of the SSM

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Figure 7

Transmission angles of the four- and five-bar loops

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Figure 8

The embodiment of the coupler link: (a) spherical triangle; (b) streamlined embodiment; (c) coupler designed based on the cross element of an off-the-shelf universal joint with an added link at its end

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Figure 9

A rotation-reversal mechanism based on SSMs: (a) single SSM; (b) array of two SSMs and their mirror images; (c) view of the pitch-roll wrist designed with the rotation-reversal mechanism

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