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Research Papers

Disturbance Response of Two-Link Underactuated Serial-Link Chains

[+] Author and Article Information
Ravi Balasubramanian1

School of Mechanical, Industrial, and Manufacturing Engineering,  Oregon State University, Corvallis, OR 97331ravi.balasubramanian@oregonstate.edu

Joseph T. Belter

Department of Mechanical Engineering,  Yale University, New Haven, CT 06520joseph.belter@yale.edu

Aaron M. Dollar

Department of Mechanical Engineering,  Yale University, New Haven, CT 06520aaron.dollar@yale.edu

1

Corresponding author.

J. Mechanisms Robotics 4(2), 021013 (Apr 25, 2012) (10 pages) doi:10.1115/1.4006279 History: Received January 05, 2011; Revised January 13, 2012; Published April 25, 2012; Online April 25, 2012

In an attempt to improve the performance of underactuated robotic hands in grasping, we investigate the influence of the underlying coupling mechanism on the robustness of underactuated hands to external disturbance. The coupling mechanisms used in underactuated mechanisms can be divided into two main classes based on the self-adaptive transmission used to route actuation to the degrees of freedom, namely single-acting and double-acting transmissions. The kinematic coupling constraint is always active in double-acting mechanisms, while there are specific combinations of external disturbances and mechanism parameters that render the constraint inactive in single-acting mechanisms. This paper identifies unique behaviors in terms of mechanism reconfiguration and variation in grasping contact forces that result from the underactuated hand’s response to external disturbance forces and show that these behaviors are a function of the coupling mechanism, actuation mode, and contact constraints. We then present an analysis of how these behaviors influence grasping ability of the hand and discuss implications for underactuated hand design and operation.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

Examples of underactuated hands: (a) single-acting cable-driven system, (b) single-acting when the coupling breaks down (cable slack), and (c) double-acting linkage-driven system

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Figure 2

A two-link revolute–revolute finger making contact with an object and simultaneously acted on by a disturbance force fe . Examples of underactuated mechanisms are shown in Fig. 1.

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Figure 3

Disturbance response of a two-link finger in (a) decoupled mode and (b) in position-control mode with a double-acting mechanism. (c) Disturbance response of a two-link finger in position-control mode in a single-acting two-link hand. The contours show variation in distal-joint deviation dθ2 as a function of disturbance force fe and its location b. The ratio of joint stiffnesses Kr  = K2 /K1 had a value of 5 in this analysis.

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Figure 4

Contact force and location combinations that nullify the pretension p in the actuator

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Figure 5

Variation of the compliance (mm/N) of an underactuated mechanism with an active coupling constraint (such as in a double-acting mechanism). The parameters explored are distal link configuration θ2 , joint-stiffness ratio Kr , the pulley radius ratio R, and disturbance force location b. The joint-stiffness ratio Kr increases from left to right across the subfigures and the joint angle θ2 increases from top to bottom across the subfigures. The disturbance force fe had a value of 5 N in this analysis.

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Figure 6

Variation of the compliance (mm/N) of an underactuated mechanism where the coupling constraint can become inactive (such as in a cable-driven single-acting mechanism in position control mode). The parameters explored are distal link configuration θ2 , joint stiffness ratio Kr , pulley radius ratio R, and distal force location b2 . The joint-stiffness ratio Kr increases from left to right across the subfigures and the joint angle θ2 increases from top to bottom across the subfigures. The thick solid line represents the parameter combinations at which the mechanism transitions into the decoupled mode. The thin lines represent parameter combinations at which the joints are coupled, and the dotted lines represent parameter combinations at which the joints are completely decoupled. The disturbance force fe had a value of 5 N in this analysis.

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Figure 7

Ratio of external force magnitude to the change in object contact normal force and actuator force for object contact on each side of the equilibrium point. The equilibrium point e is indicated by × in the inset figure and the object location constraint is proximal to the equilibrium point in the (a) and distal in (b).

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Figure 8

Grasping behaviors of a two-link finger as a function of joint deflection. The fingertip has displacement along the negative X direction in the region to the right of the red solid line.

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Figure 9

Disturbance response behavior of two-link finger in (a) force-control mode, (b) double-acting position-control mode, and (c) single-acting position-control mode

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Figure 10

Disturbance response in position-control mode of (a) a single-acting mechanism where the coupling constraint becomes inactive (cable slackens). The mechanism complies naturally with the disturbance force and curls in. (b) Double-acting mechanism showing caging behavior in response to disturbance force. (c) Double-acting mechanism showing eject behavior in response to disturbance force.

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