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Technical Briefs

A Platform for Grasp Performance Assessment in Compliant or Underactuated Hands

[+] Author and Article Information
Gert A. Kragten1

Department of BioMechanical Engineering, Faculty of Mechanical, Maritime and Materials Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlandsg.a.kragten@tudelft.nl

Just L. Herder

Department of BioMechanical Engineering, Faculty of Mechanical, Maritime and Materials Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlandsj.l.herder@tudelft.nl

1

Corresponding author.

J. Mech. Des 132(2), 024502 (Jan 20, 2010) (6 pages) doi:10.1115/1.4000761 History: Received February 02, 2009; Revised November 17, 2009; Published January 20, 2010; Online January 20, 2010

This paper presents the design and evaluation of a platform for the assessment of the performance of compliant or underactuated hands. The position of objects relative to the hand can be varied in order to assess the successful grasp region. The boundary positions of this region can be measured with a precision of 3 mm. In addition, the force required to pull an object all the way out of the hand from a stable initial configuration is measured with a precision of 0.2 N for frictionless grasping. These experiments are proposed as benchmark tests to quantify the functional performance of compliant or underactuated robotic hands. This platform considers planar symmetrical grasps, where objects can move along the line of symmetry. An innovative approach to emulate frictionless grasps of circular objects is proposed as well.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic overview of the main components of the platform. The numbering of the components corresponds with the numbering in Figs.  23456. (1) is an object; (2–3) are components to let the object freely move, see Fig. 2; (4–7) are components to pull the object out of the hand and measure the required force, see Figs.  34; (8–13) are components belonging to the underactuated finger, see Fig. 5; and (14–17) are components to actuate the finger with a constant force, see Fig. 6. X and Y are the axes of the global reference frame.

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Figure 2

Picture of a freely moving object consisting of two independently rotating disks to assess frictionless grasping, where (1a) is the upper disk in contact with the distal phalanx; (1b) is the lower disk in contact with the proximal phalanx; (2) is the linear guide; and (3) is the ruler to measure the initial and final position of the object. The underactuated finger is visible at the right side of the picture.

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Figure 3

Load cell based on a straight line guidance with leaf springs, where (4) are leaf springs of thickness t, length Lr, and height hr mounted on a distance p, according Table 1; (5) is the hole in the slider where the shaft of the object is mounted on a distance q=Lr/2 from the base; (6) is the lead screw; and (7) is the DC-motor

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Figure 4

Schematic drawing of the top view of the load cell (left side) and Wheatstone’s bridge configuration (right side) with the four strain gauges (1, 2, 3, and 4), supply voltage Vs, and voltage across the bridge Vb

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Figure 5

Underactuated finger mechanism, where (8) is the contact area of the distal phalanx of length Lph2 and with rotation ϑ2 with respect to the proximal phalanx; (9) is the distal pulley; (10) is the contact area of the proximal phalanx of length Lph1 and with rotation ϑ1 with respect to the prismatic joint; (11) is the proximal pulley; (12) is the slider of the prismatic joint; (13) is a linear spring for preloading the prismatic joint. In the upper left corner, a detail of the torsional spring at the proximal joint is shown.

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Figure 6

Constant force actuator, where (14) is the screw to adjust length a; (15) is the nearly zero-free-length spring generating a force Fs; (16) is the lever arm of length b=50 mm and rotation angle φ; and (17) is the actuation cable that applies a force Fa on the finger

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Figure 7

Measured forces to displace the object all the way out of the hand, where the repetitive measurements are shown on top of each other by different colors. The force is expressed as percentage of the actuation force of the finger. D denotes the object diameter, while (f) and (nf) denotes contact with and without friction, respectively. For experiment 4, the equilibrium positions Yeq1 (stable power grasp) and Yeq2 (unstable pinch grasp) are identified.

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