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TECHNICAL PAPERS

Equilibrium Point Control of a 2-DOF Manipulator

[+] Author and Article Information
Damien J. Clapa

 Westport Innovations Inc., Vancouver, BC, Canada

Elizabeth A. Croft2

Department of Mechanical Engineering, University of British Columbia, Room 2054-6250 Applied Science Lane, Vancouver, BC, Canada V6T 1Z4ecroft@mech.ubc.ca

Antony J. Hodgson

Department of Mechanical Engineering, University of British Columbia, Room 2054-6250 Applied Science Lane, Vancouver, BC, Canada V6T 1Z4ahodgson@mech.ubc.ca

2

Corresponding author.

J. Dyn. Sys., Meas., Control 128(1), 134-141 (Nov 28, 2005) (8 pages) doi:10.1115/1.2168474 History: Received April 02, 2005; Revised November 28, 2005

Programmable mechanical compliance in actuation is desirable for human interaction tasks and important for producing biomimetic motion, particularly for robots designed for use in domestic settings. In this paper, the equilibrium point (EP) hypothesis is proposed and implemented as a new strategy for controlling programmable compliance. The primary objective of this work is to design and demonstrate a simple robot control strategy that can potentially be used by assistive robots to learn and execute compliant interaction tasks from human demonstrations. A 2-DOF planar manipulator activated by McKibben actuators was constructed for the purpose of demonstrating the application of the EP hypothesis on an inexpensive robotic platform, such as might be used in domestic applications. The equilibrium angle and stiffness of each of the joints on the manipulator can be independently programmed. The results presented herein show stable and satisfactory tracking behavior during free motion, interaction, and transition tasks for a robot control system inspired by the EP hypothesis and implemented with a linear proportional-integral (PI) control strategy.

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Copyright © 2006 by American Society of Mechanical Engineers
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Figures

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Figure 1

Planar manipulator schematic

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Figure 2

Experimental manipulator

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Figure 3

Measured force and calculated force for an air muscle

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Figure 4

Absolute error in force between calculated and measured

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Figure 5

Schematic of planar robot controller

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Figure 6

Commanded (xEPd, heavy line) and actual (xa, light line) position versus time for free-space tests

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Figure 7

xEP error versus xa position for free-space tests

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Figure 8

x error versus x position for free-space tests

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Figure 9

Error in ky versus x position for free-space tests

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Figure 10

Commanded, equilibrium, and actual position for wiping bump. The nominal yEP is 405mm, ky is set to 1100N∕m, and vx of the equilibrium trajectory is 30mm∕s.

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Figure 11

Measured and predicted force for wiping surface forwards and backwards with a bump (top) and without one (bottom). In both cases, the yEP is 405mm, ky is set to 1100N∕m and vx of the equilibrium trajectory is 30mm∕s.

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Figure 12

Transition task: Velocity=2mm∕s, ky=1000N∕m, approach angle=30deg. Equilibrium position (top) and actual position (bottom).

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Figure 13

Measured versus predicted force normal to the wall surface for v=2mm∕s, ky=1000N∕m, and approach angle=30deg

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