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Research Papers

Modular Advantage and Kinematic Decoupling in Gravity Compensated Robotic Systems

[+] Author and Article Information
Nick Eckenstein

e-mail: neck@seas.upenn.edu

Mark Yim

Professor
e-mail: yim@grasp.upenn.edu
Department of Mechanical Engineering,
University of Pennsylvania,
Philadelphia, PA 19104

Contributed by the Mechanisms and Robotics Committee of ASME for publication in the JOURNAL OF MECHANISMS AND ROBOTICS. Manuscript received July 7, 2011; final manuscript received July 30, 2013; published online October 4, 2013. Assoc. Editor: Vijay Kumar.

J. Mechanisms Robotics 5(4), 041013 (Oct 04, 2013) (10 pages) Paper No: JMR-11-1076; doi: 10.1115/1.4025218 History: Received July 07, 2011; Revised July 30, 2013

Two new designs for gravity compensated modular robotic systems are presented and analyzed. The gravity compensation relies on using zero-free-length springs approximated by a cable and pulley system. Simple yet powerful parallel four-bar modules enable the low-profile self-contained modules with sequential gravity compensation using the spring method for motion in a vertical plane. A second module that is formed as a parallel six-bar mechanism adds a horizontal motion to the previous system that also yields a complete decoupling of position and orientation of the distal end of a serial chain. Additionally, we introduce the concept of vanishing effort where as the number of modules that comprise an articulated serial chain increases, the actuation authority required at any joint reduces. Essentially, this results in a method for distributing actuation along the length of an articulated chain. Prototypes were designed and constructed validating the analysis and accomplishing the functions of a general serial-type manipulator arm.

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References

Figures

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Fig. 2

Full gravity compensated four-bar system, showing joints and labels. Points marked with “X” correspond to P0, P1, P2, etc.

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Fig. 1

A basic gravity compensation mechanism for a rotational joint. The mass of the arm is simplified to a point mass.

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Fig. 4

Traditional arm, with same (functional) joint sequence as in four-bar system.

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Fig. 5

3D model of six-link parallel manipulator Inset: Universal joint oriented as necessary

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Fig. 3

Typical configuration of larger extended system, with simplified joint representation

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Fig. 6

Comparison of systems, simple and decomposed four-bar mechanisms and single-bar

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Fig. 7

Comparison of Kinematic Profiles

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Fig. 8

Demonstration of Inertia Calculation Principles Arcs represent curvature of path taken by the point touching the frame. All points on arms not attached to the base rotate about some point L away.

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Fig. 9

System Hanging Passively

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Fig. 10

Comparison of error for spring-pulley systems

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Fig. 11

Systems compared in Table 3 are a gravity compensated arm (top), uncompensated CKBot modules with extended links (middle), and a CKBot chain without extended links (bottom). Each circle represents a joint or CkBot.

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Fig. 12

Second Prototype extended, hanging passively

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