Analysis of the directions in which forces are applied on the hand during manual manipulation and exploration

Haptic interfaces aim at realistically simulating physical interactions within a Virtual Environment (VE) through the sense of touch. Therefore, they should display both a negligible impedance in free space and high forces and stiffness in contact. These constraints are however difficult to meet simultaneously. While compromises can usually be found when interacting through a handle grasped in hand, this is much more difficult when considering dexterous interactions with manual interfaces or exoskeletons due to the high number of degrees of freedom and limited space available. To ease their design, the authors previously made an analysis of the hand contact areas of interest for the simulation of the majority of interactions in a VE. It was shown that tracking and providing force feedback only on the five fingertips and the side of the index allows interacting naturally within a VE more than 50% of the time. This paper goes a step further and analyses in which directions these forces are applied on each hand area. A new result is that forces are required on the 6 areas of interest only normal to the skin, while tangential forces can be limited to 4 and even 1 area(s) depending on the direction.