Display of Friction in Virtual Environments Based on Human Finger Pad Characteristics

A friction display system is proposed for virtual environments. Since a user’s fingertip is often placed inside a ring or thimble of a haptic interface , the finger pad cannot move relative to the finger structure as freely as it would move in the real world. Thus, it is necessary to imitate the real world movement of the human finger pad in the virtual environment. This paper quantifies the frictional properties of the human finger pad on 9 subjects by simultaneously recording force and movement of the finger pad when it rubs against a rigid plate. Results of such tests and implementation in a virtual environment are reported. INTRODUCTION Designers of complex mechanical assemblies typically create physical prototypes to evaluate part interaction, ease of assembly, and functionality. To avoid this time-consuming and costly procedure, virtual prototyping seeks instead to employ realistic simulation and immersive interfaces. Although mechanical CAD systems provide realistic visual displays, they do not permit a designer to manipulate and interact with mechanical elements in a realistic manner. For example, a haptic interface coupled with a visual display would allow a designer to evaluate car dashboard designs [22] and to experience assembly forces [7]. In the real world, we rub on the surfaces of objects to sense their roughness, texture, discontinuity of curvature, etc. Similarly, imposing tangential forces on the users of haptic display systems creates a significant sense of realness in virtual environment. Without imposing friction in the virtual world, all objects are perceived as slippery as an icy surface. Recently, there have been a number of research works to display friction in the virtual environment. Chen et al. [1] developed a 2-D spring model by using the concept of contact area to unify the forces due to friction and adhesion. Salcudean and Vlaar [16] presented a stick-slip model, which employs a velocity threshold to define sticking, but which only employs a viscous friction during slipping. A stick-slip friction model which uses Coulomb friction in the slip phase was employed by Salisbury et al. [17]. However, these works have not included human finger pad frictional characteristics in their friction models. Unlike in the real world, in many haptic interfaces, a fingertip is placed inside a ring or thimble. Thus, the finger pad cannot be displaced relative to the thimble to create the same sense of touch as in the real world. For this reason, a haptic interface must artificially create the displacements and forces as would be created by a real frictional surface. This paper is the first attempt in integrating finger pad characteristics with the virtual friction model. Although there have been a number of recent works on the normal properties of the finger pad [6, 15, 18, 20, 21], frictional properties of the finger pad have not been explored much. Han et al. [8] investigated human fingers to mimic in making robot fingers. They found that the finger pad coefficient of friction increases significantly for small normal forces (e.g. 2N), and is relatively constant for bigger normal forces. In the field of teleoperation, Edin et al. [2] investigated the effect of slippage on relaying friction to the master operator. In the next section, a simplified basic stick-slip friction model is explained first. Then finger pad frictional characteristics are explored and incorporated in the basic model. Display of friction in virtual environments using this model is also illustrated. BASIC STICK-SLIP MODEL Assume the haptic interface is moving against a surface. In the slip phase, the friction force f f is: f f μd n v v if v vmin (1)