Technical Brief

An Underactuated PASA Finger Capable of Perfectly Linear Motion With Compensatory Displacement

[+] Author and Article Information
Eric Zheng

Department of Mechanical Engineering,
Tsinghua University,
Beijing 100084, China
e-mail: erzheng@ctemc.org

Wenzeng Zhang

Department of Mechanical Engineering,
Tsinghua University,
Beijing 100084, China
e-mail: wenzeng@tsinghua.edu.cn

1Present address: High Technology High School, Lincroft, NJ 07738.

2Corresponding author.

Contributed by the Mechanisms and Robotics Committee of ASME for publication in the JOURNAL OF MECHANISMS AND ROBOTICS. Manuscript received September 3, 2018; final manuscript received October 12, 2018; published online December 10, 2018. Assoc. Editor: Veronica J. Santos.

J. Mechanisms Robotics 11(1), 014505 (Dec 10, 2018) (8 pages) Paper No: JMR-18-1281; doi: 10.1115/1.4041786 History: Received September 03, 2018; Revised October 12, 2018

This paper presents a novel design for a robotic end effector. In particular, the design features a multifingered underactuated gripper capable of performing parallel and self-adaptive (PASA) grasping. The unique use of an eccentric cam fixed to a modified four-bar linkage mechanism allows the finger to compensate for the typical gap distance found during parallel pinching, improving the ability to grasp objects against surfaces and in tight spaces. A static analysis is performed on the design to determine the equilibrium conditions necessary for a successful grasp using this design in both the PASA modes. The mechanics of a four-bar mechanism are used to determine the grasp velocity and positioning of the hand in both grasp modes. Experimental results with a finger prototype confirm the desired closing trajectory.

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Fig. 1

Phalanx positioning during (a) parallel (pinching) and (b) self-adaptive (encompassing) modes, showing proximal and distal joint shaft angles θ1 and θ2. In the parallel phase, θ1 = θ2 to keep the distal phalanx parallel to its initial orientation.

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Fig. 2

Parallel pinching with circular motion and resulting gap distance Δs=L1−L1 cos θ, where L1 is the length of the proximal phalanx, and θ is the angle from the upright position

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Fig. 3

(a) Drawing of finger design made by CAD software and (b) diagram of the linkage mechanism, labeled by components: (1) linkage mechanism, (2) spring and mechanical limit, (3) base, (4) cam and follower, and (5) concentric gears and gear racks. (c) finger schematic during parallel pinching, with the linkage elements labeled.

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Fig. 4

Demonstration of the finger pose in the (a) upright, (b) parallel, and (c) self-adaptive cases. In the (b) parallel case, when the finger is actuated (1), the cam rotates (2), rising with respect to the finger (3) and lifting the follower (4). This turns the gears (5), thus extending the distal phalanx (6). In the (c) self-adaptive case, CE is effectively grounded by contact with the object, so when the finger is actuated (1), links BC and DE turn (2) against the spring.

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Fig. 5

Dynamic and kinematic quantities referenced in the analyses of the (a) parallel and (b) self-adaptive modes

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Fig. 6

Kinematics of a four-bar mechanism, upon which the finger is based. When a and b overlap, the new g′ is given by g′=−g.

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Fig. 7

Required motor input angle ϕ given an arbitrary phalanx positioning described by joint shaft angles θ1 and θ2

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Fig. 8

Graph of contact force F2 versus proximal joint shaft angle θ1 and distal contact distance h2 in parallel pinching mode

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Fig. 9

(a) Graph of contact force F2 versus proximal and distal joint shaft angles θ1 and θ2 during self-adaptive mode and (b) graph of contact force F1 versus proximal and distal joint shaft angles θ1 and θ2 during self-adaptive mode. In these analyses, h1 and h2 are given the average value of 20 mm.

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Fig. 10

(a–c) The prototype as it completes a parallel grasp and (d) composite overlay picture showing the entire parallel pinching process

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Fig. 11

Finger prototype in an encompassing grasp, with components from Fig. 2 superimposed. In (a–c), the finger performs a regular parallel motion, as depicted in Fig. 10. When the proximal phalanx contacts the object in frame (c), the finger begins to move self-adaptively. The grasp is complete when both phalanges touch the object, as in frame (e).



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