A Sensorless Parallel Gripper Capable of Generating Sub-Newton Level Grasping Force

Upon grasping a fragile object, small grasping force and gripper compliance are required to avoid large deformation or damage to the object. In this study, linear servomotor based mechanism is used to construct a parallel gripper which is able to achieve small gripping force and compliance without using external force sensors. The linear motor mechanism has low friction and no reduction gear. Therefore, it can produce small thrust force with high back-drivability. The proposed parallel gripper consists of two linear motor mechanisms, a guide rail, a frame, and two fingers. The thrust force of the linear motor can be controlled through a driver. The components of the gripper were manufactured using metal machining and 3D printing, and they were carefully assembled to ensure good axial alignments. Force calibration was conducted and the minimum grasping force was confirmed as a value of 0.076 N. The friction force of the guide rail was also experimentally measured and it was confirmed to be 0.090 N. Finally, grasping experiments were conducted on potato chips and tofu. Results suggested that the proposed parallel gripper is able to handle fragile objects with sub-Newton level force.