Viscoelasticity is the phenomenon of time-dependent strain and/or stress in elastic solids. Various contact interfaces with anthropomorphic end-effectors and polymeric solids found in robots and manipulators are intrinsically viscoelastic. It is therefore important to model such behavior and to study the effects of such time-dependent strain and stress on the stability and sustainability of grasping and manipulation. Various models have been proposed over the years to describe such behavior of time-dependent strain and stress. Furthermore, viscoelastic solids also display typically nonlinear elastic response. Built upon a variety of literature, a new and practical latency model is proposed in this paper for the application of contact interface involving viscoelasticity in robotics. Latency model can describe various features of viscoelastic materials, such as stress relaxation, creep, and material clock. The theoretical modeling was supported by experiments in which we found two types of relaxation, depending on the loading and unloading of grasping or contact. One type is well documented in existing literature; but the other type has not been, to our best knowledge, presented before. The proposed theory can unify both types of time-dependent relaxation responses.
