Viscoelastic contact is a type of contact which includes, in addition to linear or nonlinear elastic response, time-dependent response due to relaxation or creep phenomena that govern the contact behavior. The characteristics of the time-dependent relaxation of such a viscoelastic contact are typically exponentially decaying functions, and exponentially growing functions for creep, respectively. Such contacts can be found in anthropomorphic robotic fingers, soft materials, viscoelastic skin with rigid core, and human fingers and feet. In this paper, the nature of viscoelastic contacts is investigated, and the evolution of their friction limit surfaces and of the pressure distributions at the contact interface are studied. Two cases commonly found in robotic grasping and manipulation are discussed. Based on the modeling formulation, it is found that the two important parameters of analysis and modeling for such contacts, i.e., the radius of contact area and the profile of pressure distribution, can be chosen using proposed coupling equations as the viscoelastic contact interface evolves with time. The new contribution of this paper includes a proposal of coupling equations between the two important parameters to describe the viscoelastic contact interface, and a study of the evolution of limit surfaces for viscoelastic contact interface due to temporal dependency, and the implication on grasp stability. It is found from the evolution of limit surfaces that when normal force is applied with typical viscoelastic contacts, grasp becomes more stable as time elapses. The modeling can be applied to the design of fingertips and the analysis of robotic grasping and manipulation involving 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

In this paper, the modeling and design for frictional loads applied by robotic fingertips through soft contact interface are investigated. The dependence of the sustainable friction forces and moments due to the normal load applied on the contact area is studied. A model that considers the dependence of both the contact area and the coefficient of friction on the application of normal load is proposed. Specimens of simplified fingertips, made with different materials and shaped with different thickness of the covering soft layer, are tested in order to validate the proposed model via designed experiments. The results obtained have very important relevance on the design of robotic hands and fingers equipped with soft pads or skins, and can help on the choice of suitable features for the different layers of contact pads or skins

The first part of this paper describes the development of a humanoid robot hand based on an endoskeleton made of rigid links connected with elastic hinges, actuated by sheath routed tendons and covered by continuous compliant pulps. The project is called UB Hand 3 (University of Bologna Hand, 3rd version) and aims to reduce the mechanical complexity of robotic end effectors yet maintaining full anthropomorphic aspect and a good level of dexterity. In the second part this paper focuses on the early experiences of the UB Hand 3 in performing manipulation tasks.

In this work the static and dynamic characterization of viscoelastic pads for robotic hands is performed. A quasi-linear model, developed to describe the behavior of human hand pads and, more generally, of biological tissues, is adopted in order to overcome the problems tied to classical (linear) models, often used in the robotic field. Through experimental tests, the values of the parameters of this model have been found for two different materials (a polyurethane gel and a silicon rubber) which show a behavior similar to that of human pads and seem very suitable for robotic applications. Finally, the model has been extended with the use of digital filters, making the identification process very straightforward.

The paper describes work in progress at the University of Bologna concerning the design of a new anthropomorphic robot hand. The hand is based on the modular assembly of articulated fingers that adopt an original configuration of their structure, made with rigid links connected by elastic hinges that are coaxially crossed by flexible tendons. This innovative design is suitable to host distributed sensory equipment and continuous compliant cover, allowing a high level of anthropomorphism together with great structural simplification, reliability enhancement and cost reduction. Furthermore, the proposed solution is very flexible, as it can be adapted to many different hand configurations and is not dependent on a particular type of actuation, being compatible with future availability of any kind of artificial muscles.