Experimental quantification of gripper limits determined by product detachment in a practical set up

Abstract

The detachment of a product from a suction cup gripper is a challenge that emerged in recent years in the high-speed case packing of packaged food. In this industry suction cups are used to temporarily attach a product to a case packing robot. The detachment of products from the suction cup gripper threatens throughput. In, some cases, the industry notes robots to operate at only 40 picks per minute. This is 30 % of their maximum throughput.
This study aims to gain a better understanding of the dynamic grasping strength
of suction cups. This was done by reviewing the state-of-the-art from industry and state-of-the-practice from literature. It was found that suction cups are the best overall gripper class, and that actuation time poses the main limiting factor for other grippers to perform well in this industry.
Secondly, a six-axis force moment sensor that passes airflow for gripper actuation and without limiting the pull-out load measurement performance was designed, fabricated, and validated. With the current sensor technology, the vacuum hose must be placed over the sensor, leading to the generation of parasitic loads. Passing the airflow through the senor structure is essential to eliminate loads otherwise induced by the stiff vacuum hose. The sensor was designed using strain gauges and a Maltese cross sensor structure. Finite Element Modeling and optimization using sequential quadratic programming was used to determine the dimensions of the sensor. The sensor validation showed a maximum measurement error of 8% in the z direction.
Lastly, with the validated sensor, the dynamic grasping strength of compliant and stiff suction cups was measured using motion paths and products that are typically seen in the packaged food industry. For both the stiff and compliant suction cups, the moment around the axis perpendicular to the plane of motion showed to be a leading factor in detachment of a product from a suction cup gripper. The failure of stiff suction cups is explained by impulse loading, this results in peaks in the load in the force in horizontal direction and the moment perpendicular to the plane of motion. For the compliant suction cups, the detachment was found to be caused by accelerating downwards while the product was not in the center of the gripper. These results have led to the recommendation of two gripper designs. The first, is a suction cup-underactuated hybrid gripper. The second design intently provides rotation freedom to reduce the moment loading on the suction cup. The second design
requires input-shaping to reduce the vibrations at the end of the motion cycle.