Research Papers

SpinyHand: Contact Load Sharing for a Human-Scale Climbing Robot

[+] Author and Article Information
Shiquan Wang

Department of Mechanical Engineering,
Stanford University,
424 Panama Mall, Building 560, Stanford, CA 94305
e-mail: shiquan.wang@flexiv.com

Hao Jiang

Department of Mechanical Engineering,
Stanford University,
424 Panama Mall, Building 560, Stanford, CA 94305
e-mail: hao.jiang@flexiv.com

Tae Myung Huh

Department of Mechanical Engineering,
Stanford University,
424 Panama Mall, Building 560, Stanford, CA 94305
e-mail: taemyung@stanford.edu

Danning Sun

Department of Mechanical Engineering,
Stanford University,
424 Panama Mall, Building 560, Stanford, CA 94305
e-mail: dannings@stanford.edu

Wilson Ruotolo

Department of Mechanical Engineering,
Stanford University,
424 Panama Mall, Building 560, Stanford, CA 94305
e-mail: wruotolo@stanford.edu

Matthew Miller

Department of Mechanical Engineering,
Stanford University,
424 Panama Mall, Building 560, Stanford, CA 94305
e-mail: millerm2@stanford.edu

William R. T. Roderick

Department of Mechanical Engineering,
Stanford University,
424 Panama Mall, Building 560, Stanford, CA 94305
e-mail: wrtr@stanford.edu

Hannah S. Stuart

Assistant Professor
Department of Mechanical Engineering,
University of California,
2521 Hearst Avenue, Berkeley, CA 94709
e-mail: hstuart@berkeley.edu

Mark R. Cutkosky

Department of Mechanical Engineering,
Stanford University,
416 Escondido Mall, Building 550, Stanford, CA 94305
e-mail: cutkosky@stanford.edu

1Corresponding author.

2Present address: Flexiv Robotics, Ltd., 4500 Great America Pkwy, Santa Clara, CA 95054.

Contributed by the Mechanisms and Robotics Committee of ASME for publication in the Journal of Mechanisms and Robotics. Manuscript received December 31, 2018; final manuscript received February 4, 2019; published online April 9, 2019. Assoc. Editor: Veronica J. Santos.

J. Mechanisms Robotics 11(3), 031009 (Apr 09, 2019) (13 pages) Paper No: JMR-18-1464; doi: 10.1115/1.4043023 History: Received December 31, 2018; Accepted February 15, 2019

We present a hand specialized for climbing unstructured rocky surfaces. Articulated fingers achieve grasps commonly used by human climbers. The gripping surfaces are equipped with dense arrays of spines that engage with asperities on hard rough materials. A load-sharing transmission system divides the shear contact force among spine tiles on each phalanx to prevent premature spine slippage or grasp failure. Taking advantage of the hand’s kinematic and load-sharing properties, the wrench space of achievable forces and moments can be computed rapidly. Bench-top tests show agreement with the model, with average wrench space errors of 10–15%, despite the stochastic nature of spine/surface interaction. The model provides design guidelines and control strategy insights for the SpinyHand and can inform future work.

Copyright © 2019 by ASME
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Fig. 1

SpinyHand grasping a pumice rock. The contact areas are covered in microspines to support large tangential forces.

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Fig. 2

(Left) SpinyHand CAD rendering shows 22 spine-laden tiles in yellow distributed among four independent fingers and a palm. (Right) Rock climbing techniques (ad): (a) sloper grasp on a gently curved surface; (b) open hand/half crimp grasp on an edge; (c) pinch grasp on a narrow feature; (d) pose between a sloper grasp and a pinch, which is not commonly used by human rock climber but can be helpful for SpinyHand to envelop large curvatures.

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Fig. 3

Spine tile design: (1) sliding block for prismatic motion of the finger spine tile along x axis, (2) fixed pulley (horizontal metal rod), (3) groove for tendon routing, (4) sliding channel for palm tile, and (5) fixed pulley (vertical metal rod). The tile coordinates are consistent with the finger phalanx coordinates P.

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Fig. 4

Diagram and pictures of the finger: (1) fingernail, (2) spine tile, (3) joint pulley, (4) phalanx extension spring, (5) phalanx frame, (6) phalanx pulley, and (7) joint extension spring. Ft is the tendon force. The tendon routes around each joint pulley and phalanx pulley, so that Ft simultaneously curls the finger and slides the spine tile along the phalanx.

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Fig. 5

(Left) Palm experiment setup. (Right) Diagram of a two-tile system. Parts with similar boundaries or shades are mounted together as single moving parts. Two tiles on the schematic to the right are preloaded by springs (stiffness kp) mounted to a moving palm frame. The tiles have contact stiffness k1 and k2 with respect to the ground. A tendon routes over pulleys on the tile and palm frame. Local tendon tensions Ft1 and Ft2 may be different due to friction on pulleys between adjacent tiles. x1, x2 and x are global displacements of the tiles and palm frame, respectively.

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Fig. 6

Predicted and experimental maximum load of a two-tile system. Horizontal axis shows the contact spring configurations (springs 1, 2, and 3 have stiffness of 1.4, 2.2, and 4.9 N/mm, respectively). Five measurements are conducted for each spring configuration. The percentage shows how much friction can improve the loading capacity.

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Fig. 7

Empirical maximum load of a four-tile system with and without friction. Horizontal axis shows the contact stiffness spring configurations, with the spring type sequence starting from the left.

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Fig. 8

Three coordinates are used in the hand model: (1) palm (global) coordinates G, (2) finger coordinates F, and (3) phalanx coordinates P. Shaded dots indicate the contact locations. ci and qi are contact forces and joint angles, respectively. A third finger is behind the grasping surface and therefore displayed with dotted lines. (a) Crimp grasp with only distal phalanx contact and (b) pinch/sloper grasp with proximal contacts broken.

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Fig. 9

Floating-wrist wrench space computation procedure. (1) Compute (𝒲fi, 𝒳fi) for every finger i using the forward approach in Sec. 5.4. Point clouds of feasible wrenches for two opposed fingers are constrained by spine tile failure at any phalanx or excessive compressive normal force Fz relative to the palm frame. (Contrained to feasible values for 𝒲fi corresponding 𝒳f) calculated.) (2) Feasible grasp wrenches are searched with incremental wrist motions and constrained by hand kinematics. Determine the minimal size of search space for xg by testing xg sparsely with an increasing range until the corresponding xfi of each finger fully covers the solution space 𝒳fi. Search for every valid wg, such that its corresponding (xf1, xf2, … xfn) all stay within (𝒳f1, 𝒳f2, … 𝒳fn). Sum up every (wf1, wf2, … wfn) corresponding to the valid set of (xf1, xf2, … xfn) to obtain the points cloud that approximates the wrench space 𝒲g. (Contrained by hand kinematics for all feasible combinations of 𝒳fi.) (3) Find the outer boundary of the point cloud using convex hull algorithm, so that it can be used to solve maximum wrench limit along any given wrench direction. (Boundary of all feasible 𝒲g.)

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Fig. 10

Spine attachment force limit curve of a phalanx spine tile. Each point represents data of at least five measurements. The results are presented in polar coordinates (top) and Cartesian coordinates (bottom). Shaded regions represent compressive (positive) contact forces and are not of main interest in this study. The loading angle is defined as the angle between Fx and the loading force direction, while the maximum loading force magnitude is defined as Fx2+Fz2.

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Fig. 11

Experimental setup for measuring phalanx contact forces of a gripper with SpinyHand fingers: (1) interchangeable surface with set curvature (300 mm diameter in the photograph), (2) mounting plate for the fingers, (3) grounded board covered in PTFE film, (4) loading frame with slots to adjust the spring length, (5) signal processing board for the sensors, (6) interchangeable contact surface covered with tactile sensors and sand paper, and (7) force gauge to measure the loading force (Mark-10, Series 4). The phalanx coordinates P and global coordinates G are shown.

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Fig. 12

Variation of shear (Fx) and normal (Fz) contact forces on each of the phalanges as the gripper is loaded with increasing force along the negative global Fz direction. Model predictions in the left two plots show similar trends as the actual measurements in the right two plots.

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Fig. 13

Wrench space experiments and model predictions for grasping on a 300-mm diameter curved surface. The wrench space hull is numerically computed with the grasp model. The dots are average results of five measurements along each wrench direction vector. The sphere displays the standard deviation of loading force magnitude for each data point. The dots that are not associated with spheres are mirrored data to show the complete wrench space. Prediction error on average is 10%.

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Fig. 14

Visualization of failure modes and loading configuration on two grasping conditions (300 mm and 700 mm diameter surfaces) that have been verified with experiments. Light and dark (located only in quadrants I and III) asterisks represent proximal and distal phalanx failure (PP and DP), respectively. The dashed lines and dotted lines indicate optimal loading planes to achieve large resisting force and moment, respectively.

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Fig. 15

Shear and normal grasp force limits with changing finger tendon forces for both the 300-mm (higher curvature) and 700-mm (lower curvature) diameter surfaces. The shear force is under loading configuration of Fx = 50My.

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Fig. 16

Shear and normal grasp attachment force limits for the two previously discussed surfaces with changing middle and distal phalanx pulley radii and constant finger tendon force (46 N). The shade map shows the force magnitudes. Surface A and B are 300-mm and 700-mm diameter surfaces. The zero-force regions indicate that phalanx contact failure has occurred before applying the external load; in these regions, the dark blocks are cases where the gripper ejects itself away from the surface. The diamond is the pulley radii selections of the implemented SpinyHand.

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Fig. 17

Actuation and electronics design in the palm chamber: (a) double bushings shaft support for bottom chamber motors, (b) double bushing shaft support for top chamber motors, (c) four-layer PCB board, (d) small motor and worm drive assembly, (e) big motors, (f) mounting plates for hall effect transducers to measure rotary finger position and torque, (g) cubic magnet for torque sensing, and (h) worm gear mounted to the finger base and magnet for its position sensing. More details about the four-layer PCB board are in front view (c1): (i) sensor signal conditioning circuits, (ii) Teensy 3.2 microcontroller, (iii) power regulator, and (iv) dual channel small motor driver and in rear view (c2) where (v) are four stacked big motor drivers.

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Fig. 18

Rotary finger base design: (1) worm gear, (2) thrust bearing, (3) compact cross-roller bearing (transparent mode), (4) finger base adapting plate, and (5) finger base phalanx. The rectangular areas with dotted line show the solid part of the palm wall pinched by the finger base assembly.



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