SPINE Gripper: A Twisted Underactuated Mechanism-based Passive Mode-Transition Gripper

TL;DR

The SPINE gripper addresses the challenge of achieving stable grasping and in-hand rotation with a single actuator by embedding mechanically encoded power transmission logic into a Twisted Underactuated Mechanism (TUM). This design enables direction-invariant contraction and passive mode switching governed solely by input torque magnitude, validated through analytical modeling, extensive experiments, and real-world demonstrations. The work provides kinematic and elastic models (including $F_s$, $X(\theta)$, and $J_{\text{TUM}}$) and shows that a friction generator can set robust torque thresholds without sensors or control. The results suggest that non-coplanar, multifunctional manipulation can be realized through structural design alone, yielding robust, actuator-free end-effectors suitable for reliable manipulation in constrained or unstructured environments.

Abstract

This paper presents a single-actuator passive gripper that achieves both stable grasping and continuous bidirectional in-hand rotation through mechanically encoded power transmission logic. Unlike conventional multifunctional grippers that require multiple actuators, sensors, or control-based switching, the proposed gripper transitions between grasping and rotation solely according to the magnitude of the applied input torque. The key enabler of this behavior is a Twisted Underactuated Mechanism (TUM), which generates non-coplanar motions, namely axial contraction and rotation, from a single rotational input while producing identical contraction regardless of rotation direction. A friction generator mechanically defines torque thresholds that govern passive mode switching, enabling stable grasp establishment before autonomously transitioning to in-hand rotation without sensing or active control. Analytical models describing the kinematics, elastic force generation, and torque transmission of the TUM are derived and experimentally validated. The fabricated gripper is evaluated through quantitative experiments on grasp success, friction-based grasp force regulation, and bidirectional rotation performance. System-level demonstrations, including bolt manipulation, object reorientation, and manipulator-integrated tasks driven solely by wrist torque, confirm reliable grasp to rotate transitions in both rotational directions. These results demonstrate that non-coplanar multifunctional manipulation can be realized through mechanical design alone, establishing mechanically encoded power transmission logic as a robust alternative to actuator and control intensive gripper architectures.

Figures (10)

• Figure 1: Single actuator-based PassIve Non-coplanar degree of freedom modE transition (SPINE) gripper: Passive mode-transition gripper with mechanically encoded power transmission logic for grasp and in-hand rotation in fruit harvesting, exploiting the manipulator wrist's rotational motion without dedicated actuators or control electronics.
• Figure 2: Functional components and mechanically encoded power transmission behavior of the SPINE gripper during door handle manipulation. (a) The SPINE gripper exhibits friction-governed transition between grasping and in-hand rotation, with mode switching determined by torque thresholds. (b) Structural elements include the twisted underactuated mechanism (TUM), gripper, friction generator, and frame. (c) Approaching phase: the gripper moves toward the door handle without generating grasping force. (d) Force buildup phase: contact induces grasping force as input torque increases. (e) Rotating phase: input torque exceeding maximum static friction initiates rotation while maintaining stable grasp; grasping force remains proportional to kinetic friction torque as the system transitions to continuous rotation.
• Figure 3: Bidirectional behavior, stroke geometry, and deformation characteristics of the twisted underactuated mechanism (TUM). (a) A conventional gear differential exhibits no singularity, maintaining a fixed input-output displacement relationship. (b) The TUM contains a singularity enabling unidirectional contraction under bidirectional input rotation. (c) Strip spacing variation under twisting modeled using two-dimensional geometry. (d) Geometric and deformation characteristics relating rotation $\theta$, contraction $X$, and force projection along the $s$ axis, with bending-dominated deformation governing elastic force $F_s$.
• Figure 4: Schematic representation for kinematic modeling of the TUM.
• Figure 5: Experimental setup to evaluate kinematics, elastic force and Jacobian-based motion anlaysis of the TUM.