Data for: MOGrip: Gripper for multi-object grasping in pick-and-place tasks using translational movements of fingers

Humans utilize their dexterous fingers and adaptable palms in various multi-object grasping strategies to efficiently move multiple objects together in various situations. Advanced manipulation skills, such as finger-to-palm translation and palm-to-finger translation, enhance dexterity in multi-object grasping. These translational movements allow the fingers to transfer the grasped objects to the palm for storage, enabling the fingers to freely perform various pick-and-place tasks while the palm stores multiple objects. However, conventional grippers, although able to handle multiple objects simultaneously, lack this integrated functionality, which combines the palm's storage with the fingers' precise placement. Here, we introduce a gripper for multi-object grasping that applies translational movements of fingertips to leverage the synergistic use of fingers and the palm for enhanced pick-and-place functionality. The proposed gripper consists of four fingers and an adaptive conveyor palm. The fingers sequentially grasp and transfer objects to the palm, where the objects are stored simultaneously, allowing the gripper to move multiple objects at once. Furthermore, by reversing this process, the fingers retrieve the stored objects and place them one by one in the desired position and orientation. A finger design for simple object translating and a palm design for simultaneous object storing are proposed and validated. In addition, the time efficiency and pick-and-place capabilities of the developed gripper were demonstrated. Our work shows the potential of finger translation to enhance functionality and broaden the applicability of multi-object grasping.

Methods
Experiments

To validate the motion of the decoupling design, the grasping tendon was pulled with a motor (100:1 Micro Metal Gearmotor HPCB 6V, Pololu). The movement of five different decoupling linkages was recorded by a camera with 30 fps, and the positions of the three red markers of each frame were tracked through MATLAB (MathWorks). The MATLAB function “imfindcircle” was utilized to find the red markers, and through the positions of these three markers, the rotation angle and translation distance of the finger were calculated. Experiments for each decoupling link design were repeated five times.

The storing force along the y-axis was measured by a tensile testing machine (INSTRON 5948 Microtester). In the experiment measuring the storing force for a single object, to ensure repeatability in the process of inserting the object into the storage, the storage was divided in half and placed on both sides of the rail. Then, the target was placed between the storages, and stored by moving both storages to the center (figs. S4B and S4C). Each stored target was pulled 5 times through a tensile testing machine at a speed of 30 mm/min, and the maximum pulling force was measured as the storing force (fig. S4D). The storing force was also analyzed using finite element (FE) simulation (fig. S4E).

Similarly, for the experiment on the simultaneous storing capability of two objects, the settings from the previous experiment (Figs. S4B and S4C) were utilized to ensure the reliability of the experimental setup. The storage was divided into three parts as shown in Fig. S8A. The two objects were placed on both sides of the middle storage (highlighted in green), and the left and right storages (highlighted in orange) were moved to the center to store both objects. The black cylinder was pulled 5 times for each experiment at a speed of 30 mm/min through a tensile testing machine (INSTRON 5948 Microtester), and the maximum pulling force was measured as the storing force (Fig. S8B).

The storing force along the x-axis was measured by a load cell (KTOYO 333FDX) while pulling the objects using a linear motor (Actuonix P16-150-256-12-P) at a speed of 4.2 mm/s (Fig. S5). For repeated experiments, a weight block was hung to apply force to insert the cylindrical object into the storage while rotating the belt. When calculating the storing force, the weight of the block was subtracted from the measurement obtained by the load cell. All experiments for each parameter were repeated five times.

In the experiment measuring the placement error of the multi-object grasping sequence, the configuration of the object before grasping and after placement was captured using a camera (ABKO APC900). Blue markers were placed at the center of mass of the object and at a point 30 mm along the length from that center. The position changes of the markers were measured using OpenCV. In the storing process of the multi-object grasping sequence, the distance at which each object was stored from the entrance of the storage was set to 30 mm.

Simulation

Finite element analysis (FEA) was conducted using the FEA software ABAQUS (ABAQUS 2023, Dassault systems). All simulation conditions were set to be identical to the experimental conditions. The belt surfaces opposite to the hair were set as a fixed boundary condition, and the gravitational force applied to the hair was also considered (fig. S4E, i). In addition, by dividing the simulation steps, the object was stored in the storage by pushing the hairs (fig. S4E, ii), and after the storing process was finished, the maximum pulling force was obtained by pulling the object (fig. S4E, iii). The objects were considered as a rigid body, and all contact between the object and hairs and the hairs with each other was considered as “general contact condition.” Since dynamic motion occurs as the hair is pushed, static analysis is challenging; therefore, the simulation was conducted through dynamic analysis. For the quasi-static assumption, both mass scaling and time period were set to 1, and the kinetic energy of the hair belt was less than 3% of the internal energy at every step until the maximum storing force was measured.