Characteristics of contact and limit surface for viscoelastic fingers

Viscoelastic contact is a type of contact which includes, in addition to linear or nonlinear elastic responses, time-dependent relaxation function that governs the contact behavior. The characteristic of the time-dependent relaxation of such viscoelastic contact is typically an exponentially decaying function. Such contacts can be found in anthropomorphic soft fingers, viscoelastic skin with rigid core, and human fingers. In this paper, the nature of viscoelastic contact is investigated, as well as the resulting limit surfaces. Two cases commonly found in robotic grasping and manipulation are discussed: (i) maintaining constant area of contact after the application of normal force, and (ii) maintaining constant normal contact force after the initial contact is made. A significant yet not so intuitive result was that the two important parameters describing viscoelastic contacts can seemingly be specified independently based on theory; however, they generally follow exponentially decaying patterns. In this paper, coherent assumptions are made to correlate the two parameters. It is also found that the nature of contact interface with limit surface has a profound effect on the stability of grasping and manipulation using viscoelastic fingertips. The results can be applied to the design of fingertips and the analysis of robotic grasping and manipulation involving viscoelastic fingers.