The Role of Digit Arrangement in Soft Robotic In-Hand Manipulation

The need for robotic hands capable of gentle in-hand manipulation is growing rapidly as robots enter the real world. In this work, we show that the arrangement of digits in a soft robotic hand has a strong effect on in-hand manipulation capabilities. Introducing task-based performance metrics which quantify the range of motion, repeatability, and accuracy of in-hand manipulation tasks, we investigate hand designs with finger arrangements ranging from axisymmetric-circular to anthropomorphic. Using an open-source soft robot simulator, the effect of object size and aspect ratio on the in-hand manipulation performance is studied for a variety of finger arrangements, and findings are validated using a physical hardware platform. We found that the ideal finger arrangement is task-dependent; anthropomorphic arrangements excel at lateral translations, and axisymmetric arrangements are best suited for rotations. The aspect ratio of the object also has a strong effect on in-hand manipulation, with anthropomorphic designs performing best on objects of high aspect ratio, and axisymmetric arrangements doing well on objects of low aspect ratio. These findings are further confirmed in a real-world task with delicate pastries, where gentle in-hand manipulation is critical. Overall, our results suggest that active control of digit arrangement is necessary for soft robotic hands to maximize in-hand manipulation capabilities with arbitrary objects.