Virtual human hand: forces calculation in power or precision grasp

Abstract

The aim of this work is to show a novel environment to simulate and calculate the forces exerted in each point of contact when grasping with power or precision. Virtual hand was designed and validated with 25 degrees of freedom (DOF) without the 2 DOF for the wrist. Based on this model the finger’s contact when grasping with power is three contacts for each finger and two for the thumb. These contacts are in the proximal phalanx, middle phalanx, and distal phalanx for each finger, except the thumb, where the contacts are in middle phalanx and distal phalanx. Each contact has three forces: one normal and two frictional forces, and a torque one. When grasping with precision we only use the fingertip and depending on the size of the object we will use between two to five fingertips. Based on Geerligs' schematic representation of the different skin layers we highlight the ones that interest us most. These layers help us to define the three parts, in which each part has a Poisson's ratio and a Young's modulus. Once the position and the material of fingers are defined we create the model to calculate the forces acting in the action of grasp. Based in known normal forces applied related with the maximum displacement of the fingertip, where this maximum displacement is three millimeters and the force exerted in this maximum displacement is 6 N, we calculate the other normal forces applied in the other contacts for each finger. The interest of this study is to give to physician and physiotherapist new tools to evaluate and design new rehabilitation programs, so that the ergonomist can design and build new appliances based on the forces exerted.