Grasp Quality Evaluation in Underactuated and Compliant Robotic Hands: New Indexes and New Challenges

The new trend in the design of robotic hands is to make them underactuated and compliant, so that they can safely interact with the enviroment, and adapt to the objects they have to grasp.
Evaluating the grasping capabilities of such type of hands is a challenging task because there is the need of an evaluation method that takes into account i) which forces can be actually controlled by the hand, depending on its actuation system, and ii) the parameters that characterize the contact points such as the friction coefficient and the maximum and minimum applicable contact forces.
To this aim, the work presented in [1] revisits some traditional quality measures developed for multi-fingered, fully actuated hands, and applies them to the case of underactuated hands. Measures based on the wrench space computation, namely the largest minimum resisted wrench, and measures of contact and grasp robustness, namely the Potential Grasp Robustness (PGR) and the Potential Contact Robustness (PCR), are compared through simulated examples. Both types of indexes are found to be suitable for evaluating underactuated grasps in a realistic and coherent way, because they can account for friction constraints and physically achievable contact forces.
Underactuated and compliant hands can adapt to the shape of the objects they have to grasp and tend to perform power grasps, rather than precision grasps. This consideration lead to the work described in [2], where authors demonstrate that the PGR can be applied not only to precision grasps, but also to power grasps.
The workshop entitled “Evaluation and benchmarking of underactuated and soft robotic hands” was held at IROS 2016 to discuss on the possibility of having a common benchmarking framework for assessing the quality of compliant and underactuated manipulation systems, and highlighted that in the community there is a clear need of comparability and reproducibility, not only for soft and underactuated hands, but for general robotic grasping systems.
The posters and the slides that were presented at the workshop are available here.



[1] M. Pozzi, A. M. Sundaram, M. Malvezzi, D. Prattichizzo, and M. A. Roa, “Grasp quality evaluation in underactuated robotic hands,” in Proceedings, IEEE/RSJ International Conference on Intelligent Robots and Systems, 2016. [PDF]

[2] M. Pozzi, M. Malvezzi, and D. Prattichizzo, “On grasp quality measures: Grasp robustness and contact force distribution in underactuated and compliant robotic hands,” IEEE Robotics and Automation Letters, vol. 2, 2017. [Link]