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Active Surface-Driven Reconfigurable Gripper: Robust Grasping and Sequential Manipulation of Thin Objects
Ziyi Zheng, Keqi Zhu, Hao Wu, Yanzhe Wang, Huixu Dong
TL;DR
Thin deformable objects are difficult to grasp robustly because existing approaches often require precise control and are limited across object states. This paper introduces a reconfigurable gripper with an active-surface thumb, underactuated compliance, and a repositioning-based strategy. Experiments show robust grasping across multiple thin objects and book configurations, including long sequential “grasp-place” tasks.
Problem
Thin deformable objects require sustained contact, while existing methods often depend on precise finger gait control and limited object or state conditions.
Method
The paper develops a reconfigurable gripper with an active-surface thumb, underactuated fingers, and an active-surface strategy that repositions books before parallel grasping.
Results
The gripper robustly grasps flat thin objects, achieves a high success rate for vertically packed books, and completes long sequential “grasp-place” tasks.
Takeaways & Limitations
The study supports active surfaces as a low-control-complexity and generalizable approach for grasping and manipulating thin deformable objects.
Abstract
from arXiv · showhide
Robotic grippers face substantial challenges in grasping and manipulating thin objects. Most existing grippers rely on highly precise approach and grasp motions, which limits robustness and reduces applicability. This paper explores thin-object grasping using books as a representative example. Here, we propose a novel solution that integrates an active surface with underactuated compliance to achieve stable grasping of thin objects without complex control. First, an underactuated gripper with an active surface is designed. The active-surface thumb performs in-hand repositioning of the target book without requiring adjustments of the robot arm or the other fingers, while the underactuated fingers establish compliant contact conditions with the environment, and the reconfigurable structure enables reliable grasping of books under different configurations. Second, we establish a kinematic model of the gripper, and determine the initial grasp postures for two representative scenarios (books lying flat on a desktop and books vertically packed in a shelf). Third, by analyzing the physical model of a book lying on a table and its interaction with the gripper and the environment, we systematically optimize the structural parameters and grasping strategy. Finally, extensive experiments validate the effectiveness of the proposed gripper and strategy. The results demonstrate strong robustness and adaptability when grasping thin objects placed flat (including books, paper, fabric, plastic film, and mouse pad), as well as a high success rate when grasping vertically packed books. Moreover, the proposed gripper can reliably complete long sequential "grasp-place" tasks.
I. INTRODUCTION
Thin deformable objects are difficult to grasp robustly because they require sustained contact and precise control across varied configurations. The paper proposes an active-surface, reconfigurable gripper and strategy to address these challenges for books and other thin objects.
- Thin deformable objects are difficult to grasp because they deform easily and require grippers to maintain surface contact during grasping.
- Books provide a representative challenge because they have separable layers and occur both flat on surfaces and vertically on shelves.
- Existing approaches often require precise finger gait control, accurate models, large-sample learning, or object-specific surface properties.
- The proposed gripper combines an active-surface thumb, reconfigurable mechanisms, underactuated joints, and simple control for two book states.
- Its strategy determines an initial configuration, repositions the object with the active surface, and then performs parallel grasping without complex finger gait control.
- Experiments evaluate stable grasping of separated and single-layer thin objects and completion of long sequential “grasp-place” tasks.
III. GRIPPER DESIGN
The gripper is a reconfigurable three-finger mechanism designed for both flat and vertically arranged books. An active belt thumb, wedge-shaped fingertips, and decoupled transmissions support repositioning, separation, and stable grasping.
- The gripper uses an active surface only on the thumb, while the other two fingers have wedge-shaped tips for thin-object repositioning.
- Two coupled fingers switch symmetrically between grasping and separation modes, enabling handling of books in flat and vertical states.
- Only three motors drive reconfiguration, finger opening and closing, and active-surface rotation.
- A parallel four-bar linkage provides parallel grasping, while the finger structure supports reconfiguration through motor-driven gearing.
- A silicone-coated belt supplies continuous thumb rotation and a larger contact area for repositioning and stable post-grasp contact.
- Two independent transmission layers decouple finger grasping from reconfiguration, with separate gear sets driven by separate motors.
IV. KINEMATICS ANALYSIS OF THE GRIPPER
The kinematic analysis models the gripper’s grasping and reconfiguring motions and derives endpoint positions from coordinate transformations. It also defines initial configurations and compares strategies for flat and shelf-packed books.
- Grasping uses coordinated crank-slider and parallel four-bar motions, whereas reconfiguration is directly driven through a planetary gear set.
- The kinematic model defines gripper, proximal-joint, distal-joint, and slider coordinate systems for describing finger geometry.
- Homogeneous transformations map the gripper coordinate frame to endpoint F and provide its coordinates for kinematic analysis.
- The grasping-strategy analysis specifies an initial configuration for books lying on a surface and compares two strategies for bookshelf books.
- Endpoint Q coordinates are derived analogously within the gripper coordinate frame.
V. GRASPING STRATEGY AND PARAMETER OPTIMIZATION
The gripper determines an initial configuration from book thickness and fingertip spacing, then uses passive finger adaptation and active-surface repositioning to manipulate the book without further gripper adjustments.
- The thumb presses the book while underactuated fingers passively adjust their rotation to conform to the surface.
- The fingertip angle is passively adjusted by environmental constraints through the spring-connected four-bar linkage.The relation is θ4 = ψ − α, where ψ is the gripper rotation angle and α is the fingertip angle.
- The gripper models fingertip and thumb-tip positions to obtain the book thickness h and horizontal spacing s.The thumb geometry uses e for the base-to-thumb distance and l8 for thumb length.
- Once h and s are determined, the initial gripper configuration θ2 and ψ can be obtained before repositioning and grasping.
2) Grasping Strategy for Vertically Arranged Books:
For vertically arranged books, the gripper inserts wedge-shaped fingertips into the neighboring gap, separates the target book, repositions it with the active surface, and then switches to grasping mode. For flat-book repositioning, beam and force-equilibrium analysis guides parameter selection, favoring a 20° fingertip angle and approximately 40 mm finger opening.
- 2) Grasping Strategy for Vertically Arranged Books:: The gripper separates a vertically packed target book by inserting wedge-shaped fingertips into the gap between neighboring books.
- 2) Grasping Strategy for Vertically Arranged Books:: After insertion, the active surface repositions the book, the fingers switch from separation to grasping mode, and the high-friction surface stabilizes contact.
- B. Optimization of Structural Parameters and Grasping Strategies: During flat-book repositioning, the thumb must satisfy Ff2 ≤ µ2F2 while the book transitions from table-only contact to contact with the fingers.
- B. Optimization of Structural Parameters and Grasping Strategies: The contact-force model uses book geometry, friction, bending moment, and beam deformation to characterize forces throughout quasi-static repositioning.The analysis applies an Euler-Bernoulli beam model as a first-order approximation for slender books with relatively small deformation.
- B. Optimization of Structural Parameters and Grasping Strategies: The beam analysis represents the book’s deflection with a fourth-order curve determined from distributed loading and boundary conditions.The model uses Young’s modulus E, moment of inertia Iz, and four integration parameters A, B, C, and D.
- B. Optimization of Structural Parameters and Grasping Strategies: 20° fingertip angle and approximately 40 mm finger opening were selected because larger α and smaller s substantially increase the repositioning friction force Ff2.The strategy therefore favors larger spacing within the gripper’s effective range.
A. Grasping Books on the Desktop
Desktop experiments use a staged grasping strategy in which the gripper approaches a flat book, repositions it with the active surface, and then completes the grasp. Across four books, the thinner three were grasped without errors, while the thickest book had two failures during initial contact.
- A. Grasping Books on the Desktop: The desktop sequence moves the gripper downward, engages the active surface, repositions the book, and completes the grasp.The experiment used four sequential steps with simple control inputs.
- A. Grasping Books on the Desktop: The thinner three books were grasped without errors across 20 trials per book.The experiments tested four books with different sizes and surface friction coefficients.
- A. Grasping Books on the Desktop: The thickest book failed twice when it first contacted the fingers because its bending stiffness was high.The reported failures occurred at the initial finger-contact stage.
B. Grasping Books on the Bookshelf
The gripper handles vertically packed books and other extremely thin objects through active-surface repositioning and reconfigurable grasping. It also supports bidirectional book placement, while future work targets perception and broader object compatibility.
- B. Grasping Books on the Bookshelf: Bookshelf grasping inserts the fingers beside the target book, uses the thumb belt to pull it inward, then changes configuration to grasp it.The four-step procedure uses the same active-surface speed as the desktop task.
- B. Grasping Books on the Bookshelf: Few bookshelf trials failed, and no failures occurred after successful finger insertion.The reported insertion failures were attributed to finger-position deviations under open-loop control.
- B. Grasping Books on the Bookshelf: The gripper grasped plastic film, fabric, A4 paper, and mouse pad without errors, including objects as thin as 0.08mm.The tested objects differed in friction and softness, and the same strategy used for books was applied.
- B. Grasping Books on the Bookshelf: The proposed strategy completes both desktop-to-bookshelf and bookshelf-to-desktop grasp-place sequences without opening the fingers during bookshelf placement.Reverse active-surface rotation pulls the book out while it remains held, using low-friction fingertips and a high-friction active surface.
- B. Grasping Books on the Bookshelf: The active surface provides a continuously changing contact interface, while reconfiguration supports books in different states with sequential, uncoupled control.The conclusion frames the approach as low-control-complexity and generalizable for thin deformable objects.
- B. Grasping Books on the Bookshelf: Future work will add vision for automated long-sequence tasks and adjustment mechanisms to handle more thin deformable objects.The authors also plan to integrate active surfaces with dexterous hands.
Appendix
The appendix formulates the gripper’s kinematics through coordinate transformations and mechanism constraints. These relations determine fingertip positions and link the slider input to finger opening and closing.
- Appendix: Transformation matrices compose coordinate frames from the gripper base to linkage endpoints and finger tips.The appendix derives positions for endpoints F, G, P, and Q in the gripper coordinate frame.
- Appendix: The appendix also reports the endpoint-coordinate formulation for the gripper’s coupled linkage geometry.The transformation product from the gripper frame to endpoint F is explicitly defined.
- Appendix: The slider position is expressed from the initial distance h0, screw lead S, and screw rotation angle α.This relation connects screw actuation to the slider coordinate.
- Appendix: The planar crank-slider constraints yield equations involving slider distance h and finger angle θ2.Once h is specified, θ2 can be calculated from the derived relations.
B-1 Parameter Optimization
Parameter optimization analyzes how the repositioning geometry affects the available friction force. The selected initial distance should avoid the small-s range, where the maximum Ff2 decreases rapidly as s increases.
- B-1 Parameter Optimization: With a 10N thumb normal force, the analysis plots Ff2 under different distances s and fingertip angles α.The curves compare four fingertip angles and distance-dependent friction-force limits.
- B-1 Parameter Optimization: For s ∈(0, 30), the maximum Ff2 rapidly decreases as s increases across the evaluated fingertip angles.At larger s values, the decrease becomes more gradual.
- B-1 Parameter Optimization: The initial configuration should avoid small s values so book repositioning is less likely to enter a failure region.This recommendation follows the reported change in the Ff2 decrease rate.
B-2 Gripper Prototype
The gripper prototype combines a modular, belt-based active thumb with mechanically coupled fingers and was evaluated through friction characterization and flat-book grasping trials.
- Prototype construction: The prototype uses a 1 mm silicone-covered S3M timing belt as the thumb’s active surface.The housing was 3D-printed, while linkages and fingertips were manufactured from aluminum.
- Prototype construction: Three motors are mounted on the gripper base, whose modular design allows separate assembly of the base, fingers, and thumb.
- Experimental characterization: The friction platform pulls each object at constant speed while recording steady pulling force, using µ = Fm/mg to calculate friction coefficient.Objects were tested flat on three different surfaces.
- Grasping experiments: Flat books were grasped from the non-bound side, where separable pages make interlayer sliding more likely during grasping.Only some pages may successfully undergo subsequent repositioning after finger contact.
- Grasping experiments: 100% success was achieved across 20 trials for each of four books placed flat on a desktop.The authors attribute this performance to the small-angle wedge-shaped fingertips, which separate extremely thin objects from the table.