A Robotic End-Effector for Grasping Sacks

This article describes an end-effector and a method for manipulating 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, comprises two parallel rollers with gripping sur-faces, where the rollers are pushed towards each other. When the end-effector comes onto 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. When sufficient material is caught between the rollers, the rollers stop rotating, whereby the rollers then hold the graspable portion of the object allowing the object to be maneuvered by the robot. When the rollers turn outwardly, the object will be released. The end-effector described here has been evaluated and proven to be effective in grabbing and holding postal sacks since it can grab and hold filled sacks from any point on the sack and regardless of the sack orientation and position. This article describes the underlying principles of the design and grasp control of the end-effector 1. Background on Maneuvering Sacks Postal services across the world use sacks to hold letters, magazines and small boxes. These sacks which are handled manually by mail handlers are usually filled with magazine bundles, envelopes and parcels, and weigh up to 70 lbs. In general, several factors contribute to awkward and uncomfortable sack handling process for mail handlers: • the heavy weight of the sacks; • the lack of handles, eyelets or any other helpful operator interface on the sacks and parcels; • unpredictable shape, size, and weight of the sacks. During repetitive pick and place maneuvers, the above elements have increased the risk of wrist, finger, and back injuries among mail handlers. Figure 1 shows a postal distribution center in San Francisco where thousands of sacks are unloaded off a large slide and either emptied directly onto conveyor belts, or loaded onto carts by hand. The sacks are often very heavy and have no operator interface of any kind, which makes them difficult to grasp. This makes the process very slow and inefficient. Robotic systems with special end-effectors can be installed from above to load the sacks onto either the nearest conveyor belts or onto carts. There are very few limitations; the slide and conveyor belts are clear from above, and the carts used to receive sacks are open on top. Figure 1: A postal distribution center where thousands of sacks are unloaded off a large slide. To decrease the risk of injuries and to expedite mail processing, the US Postal Service (USPS) has employed various robotic devices to automate some of its mail handling systems. This paper describes an end-effector that is designed to work with these robotic systems to grab and hold sacks. The followings are the specifications associated with the end-effector: • The end-effector must grab and hold a sack regardless of the shape and size of the sack from any point on the sack (i.e., the end-effector should not need a gathered and flattened edge of the sack or it should not need to have the sack to be in a particular orientation prior to grasp). The mail sacks have no handles, eyelets or other operator interface, and they come in a variety of shapes and sizes, but they weigh less than 70 pounds. • The robot and the end-effector must grasp and manipulate six (6) sacks per minute for four hours without any drop. This places a hard constraint on the grasp robustness of the end-effector. Any time that the end-effector drops a sack, an operator needs to enter the robot cell for recovery, which leads to process downtime due to cell shutdown and robot initialization. Considering our difficult design requirements on grasp Proceedings of ICAR 2003 The 11th International Conference on Advanced Robotics Coimbra, Portugal, June 30 July 3, 2003