Novel Nonlinear Elastic Actuators for Passively Controlling Robotic Joint Compliance

[+] Author and Article Information
Shane A. Migliore

Laboratory for Neuroengineering,  Georgia Institute of Technology, Atlanta, GA 30332migliore@ece.gatech.edu

Edgar A. Brown

Laboratory for Neuroengineering,  Georgia Institute of Technology, Atlanta, GA 30332ebrown@ece.gatech.edu

Stephen P. DeWeerth

Laboratory for Neuroengineering,  Georgia Institute of Technology, Atlanta, GA 30332steve.deweerth@ece.gatech.edu

J. Mech. Des 129(4), 406-412 (Apr 06, 2006) (7 pages) doi:10.1115/1.2429699 History: Received August 25, 2005; Revised April 06, 2006

The ability to control compliance of robotic joints is desirable because the resulting robotic mechanisms can adapt to varying task requirements and can take advantage of natural limb and joint dynamics. The implementation of controllable compliance in robots, however, is often constrained by the inherent instability of active compliance methods and by the limited availability of the custom, nonlinear springs needed by passive compliance methods. This work overcomes a major limitation of passive compliance by producing designs for two novel mechanisms capable of generating a wide variety of specifiable, nonlinear elastic relationships. One of these designs is physically implemented as a quadratic “spring” and is used to create a passively compliant robot joint with series-elastic actuation. A simple feed-forward algorithm is then experimentally shown to be sufficient to control independently and simultaneously both joint angle and joint compliance, regardless of the presence of external forces on the joint. We believe that this is the first physically constructed system to use antagonistic quadratic springs to successfully demonstrate open-loop, independent, and simultaneous control of both joint angle and joint stiffness. Because this approach better emulates the underlying joint mechanics used by animals, it may improve both the quality and variety of robotic movements.

Copyright © 2007 by American Society of Mechanical Engineers
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Figure 1

A schematic view of a rotational joint using antagonistic series-elastic actuation. The angles α and β, of the two dc servos (top) control both the angle, θ, of the robotic joint (bottom) and the amount of spring co-contraction.

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Figure 5

A single DOF robotic joint with antagonistic series-elastic actuation provided by two expanding contour devices

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Figure 6

The accuracy with which the joint was actuated over a range of stiffness values. As stiffness increased, the variability of the actuation decreased substantially.

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Figure 7

Variations in joint stiffness produced by changing the level of co-contraction at θc=0deg. Note that as Sc increases, the slope of the torque-angle line (i.e., Sm) also increases.

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Figure 8

The relationship between commanded and measured joint stiffness. Five of the points in this plot represent the slopes of the linear regressions shown in Fig. 7. The remaining ten points come from similar experiments that were performed at θc=±45deg. The dashed trace represents the ideal joint performance with identically matched springs.

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Figure 2

The expanding contour design of a nonlinear elastic mechanism. When a force is applied, rollers are forced along a nonlinear contour and stretch is applied to a pair of linear springs (one is not visible in background).

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Figure 3

The cam-roller design of a nonlinear elastic mechanism. The top cam is attached to a motor, and the bottom cam is attached to the joint such that when either rotates, the rollers are forced to move vertically and produce stretch in a linear spring. Antagonistic actuation is provided by an identical set of cams with opposite orientation (not visible in background). In the blowup, β is the angle the line tangent to the cam at point A makes with the x axis.

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Figure 4

Implementation of the expanding contour design with a quadratic force–length relationship

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Figure 9

Compensation for an externally applied joint torque. The top three traces show the effect of noncompensated external torques on θm, while the bottom three traces show the results of the same experiment when compensation was applied. When Sc>0.28mNm∕deg, the joint was within its operating region, and when Sc<0.28mNm∕deg, one of its actuation cables went slack, producing significant angle error.



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