A Robotic Mechanism for Grasping Sacks

Abstract

This paper describes a novel robotic end-effector and a method for grasping deformable objects with undefined shapes and geometry, such as sacks and bags. The first prototype end-effector, designed for applications in the U.S. Postal Service, is comprised of two parallel rollers with gripping surfaces in which the rollers are pushed toward each other. When the end-effector comes into contact with any portion of the deformable object, the rollers turn inwardly so that a graspable portion of the object is dragged between the rollers. The rollers stop rotating when a graspable portion of the material/object is caught in between, allowing the object to be maneuvered by the robot. The object is released when the rollers turn outwardly. The end-effector described in this article can grab and hold filled sacks from any point on the sack, regardless of the sack's orientation. Experimental evaluation of the end-effector has proven the design and implementation remarkably effective. This article describes the hardware, control method, and design issues associated with the end-effector. Note to Practitioners - Delivery and postal services around the world currently use sacks to hold letters, magazines, and small boxes. The considerable weight of these sacks, their lack of handles, eyelets, or other operator interfaces, and the unpredictable shape and size of the packages within create awkward and uncomfortable handling predicaments for mail handlers at all U.S. Postal Service distribution centers. Currently, no robotic hand or end-effector is commercially available to grab and hold sacks effectively, so sacks must be handled manually by postal employees in distribution centers. This paper describes the design of a novel robotic end-effector for manipulating deformable objects with undefined shapes, such as sacks and bags. This device, which does not mimic human hand architecture, is simple and practical; it makes use of the friction between two rotating rollers to grab sack material when it is in close proximity of the end-effector. The rollers cease rotation when sufficient sack material is collected between the rollers to support the sack. They reverse their rotation, and turn outwardly to release the sack again. This end-effector is able to grab a sack at any point, does not require the edge of the sack to be gathered and flattened prior to grasp, does not require the sack to be placed on its bottom, needs no operator intervention, does not use the weight of the sack to lock and secure the sack in the end-effector, and does not damage the sack contents.