In this study, we propose a novel gripper design that passively maintains a constant grasping force without requiring electronic sensors or feedback control. Drawing inspiration from the Christie suspension system, we developed a pseudo-constant-force mechanism using a coil spring and a single-link structure. By applying this mechanism to the fingertips of a pneumatic gripper, we constructed a device capable of maintaining a consistent grasping force across varying object displacements. Additionally, we introduced a method to adjust the target grasping force by replacing the coil spring with a pneumatic spring. Pneumatic springs allow variable stiffness by altering the internal air pressure, enabling simple and precise force control through a regulator. Experimental validation demonstrated the effectiveness of the proposed gripper. A prototype using a coil spring achieved near-constant force with minimal deviation. Furthermore, incorporating 3D-printed pneumatic springs enabled dynamic adjustment of the grasping force. The pneumatic spring stiffness varied linearly with the applied pressure, although nonlinearity and hysteresis were observed at larger displacements. Despite minor discrepancies due to friction and manufacturing variances, the gripper exhibited stable and adjustable constant-force behavior, proving its suitability for handling soft or fragile objects in industrial applications.
MATSUNO et al. (2026) studied this question.