The ability of underactuated hands to grasp small objects is very limited, because the precision grasp is normally unstable. The goal of this paper is to achieve stable precision grasps by means of simple design modifications of the distal phalanges of the fingers. These modifications comprise the curving of the contact area of the distal phalanx, the application of a mechanical limit to prevent hyperextension of the distal phalanx, and the application of a compliant joint between the proximal and distal phalanges. A model is developed to calculate the limits of the finger dimensions in order to achieve stable precision grasps for different object sizes. An experimental setup is used to test the grasp stability and to verify the calculated results. It is concluded that stable precision grasps exist for the combination of concavely curved distal phalanges with a mechanical limit or with a compliant joint, if the limits to the finger dimensions are satisfied.

To evaluate and optimize the mechanical design of underactuated hands, a benchmark test is defined that quantifies the ability to hold grasped objects subject to force disturbances as occurs during for instance pick and place operations. The ability to hold is quantified by the magnitude of the maximum permitted static force on the center of a cylindrical object of known radius at which it can still be hold within the underactuated hand. A static grasp model is developed to efficiently calculate the effect of design parameters of the hand on this maximum force. This model is applied to a planar, underactuated hand with six degrees of freedom, calculating the maximum force while the rotational joint stiffnesses and distance between the fingers was varied. We demonstrated that the rotational joint stiffness has no effect on this force, while decreasing the distance between the fingers has a negative effect. These results were validated by measurements in an experimental setup. These preliminary results show that the developed benchmark test can be effective to evaluate and optimize the performance of underactuated hands to hold objects in pick and place operations.