A soft-contact model for computing safety margins in human prehension

Abstract

The soft human digit tip forms contact with grasped objects over a finite area and applies a moment about an axis normal to the area. These moments are important for ensuring stability during precision grasping. However, the contribution of these moments to grasp stability is rarely investigated in prehension studies. The more popular hard-contact model assumes that the digits exert a force vector but no free moment on the grasped object. Many sensorimotor studies use this model and show that humans estimate friction coefficients to scale the normal force to grasp objects stably, i.e. the smoother the surface, the tighter the grasp. The difference between the applied normal force and the minimal normal force needed to prevent slipping is called safety margin and this index is widely used as a measure of grasp planning. Here we define and quantify safety margin using a more realistic contact model that allows digits to apply both forces and moments. Specifically, we adapt a soft-contact model from robotics and demonstrate that the safety margin thus computed is a more accurate and robust index of grasp planning than its hard-contact variant. Previously, we have used the soft-contact model to propose two indices of grasp planning that show how humans account for the shape and inertial properties of an object. A soft-contact based safety margin offers complementary insights by quantifying how humans may account for surface properties of the object and skin tissue during grasp planning and execution.

Decoupled Control of Grasp and Rotation Constraints During Prehension of Weightless Objects

“…The other two constraints had a few options to satisfy the particular conditions and to gauge the strength (i.e., absolute r or r 2 value) and direction (i.e., negative or positive sign of r value) of correlation. A few studies have claimed that the organization of the tangential forces is constrained by the organization of the normal force, whereas the experimental results in a particular circumstance demonstrated the possibility of active control of tangential forces (Singh & Ambike, 2017;Shim & Park, 2007;Song et al, 2021). If gravity is close to zero, the role of the tangential forces should be changed such that the role of the resistant force to compensate for the weight of the handheld object is eliminated, whereas the contribution to the rotational equilibrium is still valid.…”