Balloon Pin-Array Gripper: Modeling of Holding Force Generation Mechanism and Parametric Grasping Analysis

This study presents both theoretical and experimental investigations of the shape-dependent grasping performance of the "balloon pin-array gripper," which integrates a pin-array structure with flexible inflatable balloons. First, we develop a theoretical model that captures the nonlinear expansion behavior of the balloon membrane and formulates the mechanism of holding force generation when multiple balloons simultaneously contact an axisymmetric object. Each balloon is assumed to expand until it reaches a specified compression depth, with the difference between internal pressure and membrane tension modeled as the contact pressure, which generates normal and frictional forces. Subsequently, we introduce a parametric evaluation method using axisymmetric objects, such as cylinders, conical frustums, and spheres. By systematically varying geometrical parameters, such as the height, radius, and base angle, we experimentally measure the holding force and analyze its dependence on contact area, surface orientation, and pin-array discreteness. The experimental results show reasonable agreement with the theoretical predictions with an overall coefficient of determination of R 2 = 0.63. We also demonstrate that the gripper can establish contact and generate holding force even on challenging geometries, such as downward-facing surfaces. However, several simplifying assumptions in the model—such as the omission of balloon stress distribution and balloon-to-balloon interference—are presented and discussed as limitations. Future improvements are expected to involve pressure distribution measurements and finite-element analysis. These findings provide valuable insights into the grasping mechanisms of pneumatically actuated grippers and serve as a basis for optimizing design parameters and evaluating scaling limitations.