Paolo Tiezzi

Development of UB Hand 3: Early Results

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.

UBH 3: an anthropomorphic hand with simplified endo-skeletal structure and soft continuous fingerpads

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.

Modeling of Viscoelastic Contacts and Evolution of Limit Surface for Robotic Contact Interface

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

Modelling and identification of soft pads for robotic hands

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.

Effect of layer compliance on frictional behavior of soft robotic 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

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

Experimental analysis of soft fingertips with internal rigid core

The paper describes an experimental activity concerning the mechanical characterization of soft fingertips for robotic hands, focusing on the effects of the presence of an inner rigid core, as in robotic fingers with an endo-skeletal articulated frame covered by an external layer of soft material. By means of a purposely-designed test equipment, fingertips with differently sized cores have been tested, showing the influence of the thickness of the compliant layer on the resultant fingertip stiffness with respect to different materials and loading conditions (pure normal load, normal and tangential load, normal load and torque)

Effect of Layer Compliance on Frictional Behavior of Soft Robotic 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.

Nonlinear Modeling and Experimental Identification of Hemispherical Soft Pads for Robotic Manipulators

In this work the dynamic contact of hemispherical indenter covered by a thick viscoelastic layer and pressed against a flat rigid surface is modelled. The goal is to investigate the dynamic behavior of robotic fingertips composed by an inner rigid structure covered by a soft layer, mimicking the human biological model. A quasi-linear model, frequently used to describe the behavior of soft and pulpy biological tissues, is adopted in order to achieve a compromise between the simplicity of classical linear models and the difficulty of nonlinear approaches. Two different materials (a polyurethane gel and a silicon rubber) have been experimentally tested in order to find the parameters of the model and to validate it. Finally, the advantage of the use of digital filters method in identification process is exploited, so that the identification becomes suitable for online process.Copyright

A Methodological Approach to Design of Soft Fingertips for Humanoid Robot Hands

This paper first describes the general lines of a methodological approach to design of soft fingers for humanoid robot hands. The method takes into consideration both functional aspects (related to grasp robustness and stability) and reliability aspects (as resistance to wear and tear, manufacturability, structural integratability, etc.). Then, some experimental results related to the evaluation of functional indexes are presented and discussed. Experiments have been performed on fingertips made of an inner rigid core covered by a soft layer, in order to simulate the behaviour of biological tissues covering an endo-skeletal structure. Effects of variables like thickness and hardness of material of the compliant layer have been investigated under different load conditions, to study their influence on the performance indexes.