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     

Adaptive neuro-fuzzy modeling of a soft finger-like actuator for cyber-physical industrial systems

Soft robotics is a trending area of research that can revolutionize the use of robotics in industry 4.0 and cyber-physical systems including intelligent industrial systems and their interactions with the human. These robots have notable adaptability to objects and can facilitate many tasks in everyday life. One potential use of these robots is in medical applications. Due to the soft body of these robots, they are a suitable replacement for applications like rehabilitation and exoskeletons. In this paper, we present the neuro-fuzzy modeling of a soft pneumatic finger-like actuator. This actuator is a fiber-reinforced soft robot with the shape and dimensions of a real finger and moves in planar motion. A bending sensor is used as a feedback for curvature motion of this actuator. In order to model this actuator, an adaptive neuro-fuzzy inference system is utilized to overcome the hardship in the modeling of the nonlinear performance of the soft materials. An experimental setup is designed to obtain suitable input–output data needed for modeling. The results show the applicability of the utilized method in the modeling of the soft actuator.

     

Software toolkit for modeling,simulation, and control of soft robots

The technological differences between traditional robotics and soft robotics have an impact on all of the modeling tools generally in use, including direct kinematics and inverse models, Jacobians, and dynamics. Due to the lack of precise modeling and control methods for soft robots, the promising concepts of using such design for complex applications (medicine, assistance, domestic robotics, etc.) cannot be practically implemented. This paper presents a first unified software framework dedicated to modeling, simulation, and control of soft robots. The framework relies on continuum mechanics for modeling the robotic parts and boundary conditions like actuators and contacts using a unified representation based on Lagrange multipliers. It enables the digital robot to be simulated in its environment using a direct model. The model can also be inverted online using an optimization-based method which allows to control the physical robots in the task space. To demonstrate the effectiveness of the approach, we present various soft robots scenarios including ones where the robot is interacting with its environment. The software has been built on top of SOFA, an open-source framework for deformable online simulation and is available at https://project.inria.fr/softrobot/.     

Soft medical robotics: clinical and biomedical applications,challenges, and future directions

Bioinspired soft robotics allow for safer clinical interactions with human patients but conventional, hard robots, which are often built with rigid materials and complex control systems, compromise tissue integrity, freedom of movement, conformability, and overall human bio-compatibility. Soft, compliant materials intrinsically reduce mechanical complexity, accommodate their usage environment, and provide great practical potential for medical device developments. Previous review papers have generally covered the topics of materials, manufacturing processes, actuator modeling and control, and current trends. Here, we focus on recent developments in soft robotic applications for the medical field including advances in cardiac devices, surgical robots, and soft rehabilitation and assistance devices. In medical applications, soft robotic devices not only expedite the evolution of minimally invasive surgery but also improve the bio-compatibility of rehabilitation and assistance devices. Here, we evaluate design requirements, mechanisms, achievements and challenges in these key areas. Of particular note, this paper concludes with a discussion on advances in 3D printing and adapting neural networks for modeling and control frameworks that have facilitated the development of faster and less expensive soft medical devices.     

Soft Robotics: Morphology and Morphology-inspired Motion Strategy

Robotics has aroused huge attention since the 1950s. Irrespective of the uniqueness that industrial applications exhibit, conventional rigid robots have displayed noticeable limitations, particularly in safe cooperation as well as with environmental adaption. Accordingly, scientists have shifted their focus on soft robotics to apply this type of robots more effectively in unstructured environments. For decades, they have been committed to exploring sub-fields of soft robotics (e.g., cutting-edge techniques in design and fabrication, accurate modeling, as well as advanced control algorithms). Although scientists have made many different efforts, they share the common goal of enhancing applicability. The presented paper aims to brief the progress of soft robotic research for readers interested in this field, and clarify how an appropriate control algorithm can be produced for soft robots with specific morphologies. This paper, instead of enumerating existing modeling or control methods of a certain soft robot prototype, interprets for the relationship between morphology and morphology-dependent motion strategy, attempts to delve into the common issues in a particular class of soft robots, and elucidates a generic solution to enhance their performance.      

Novel bio-inspired variable stiffness soft actuator via fiber-reinforced dielectric elastomer,inspired by Octopus bimaculoides

Intelligent Service Robotics - Compliant devices are used in a wide variety of applications like soft robots. Although soft robotics have played an important role in providing the desired...     

Soft robot review

Soft robots are often inspired from biological systems which consist of soft materials or are actuated by electrically activated materials. There are several advantages of soft robots compared to the conventional robots; safe human-machine interaction, adaptability to wearable devices, simple gripping system, and so on. Due to the unique features and advantages, soft robots have a considerable range of applications. This article reviews state-of-the-art researches on soft robots and application areas. Actuation systems for soft robots can be categorized and analyzed into three types: variable length tendon, fluidic actuation, and electro-active polymer (EAP). The deformable property of soft robots restricts the use of many conventional rigid sensors such as encoders, strain gauges, or inertial measurement units. Thus, contactless approaches for sensing and/or sensors with low modulus are preferable for soft robots. Sensors include low modulus (< 1 MPa) elastomers with liquid-phase material filled channels and are appropriate for proprioception which is determined by the degree of curvature. In control perspective, novel control idea should be developed because the conventional control techniques may be inadequate to handle soft robots. Several innovative techniques and diverse materials & fabrication methods are described in this review article. In addition, a wide range of soft robots are characterized and analyzed based on the following sub-categories; actuation, sensing, structure, control and electronics, materials, fabrication and system, and applications.     

Soft Interaction Between Body Weight Support System and Human Using Impedance Control Based on Fractional Calculus

In recent years, research and development have been conducted on robots designed to assist people with disabilities in daily activities. There is a great demand for control technology for realizing flexible contact and cooperative behavior. We here report a novel impedance control method based on a fractional calculation inspired by the viscoelastic properties of biomaterials such as muscle. This paper presents an evaluation of this concept by simulation and by experiment using a robotic system for body weight support. The experimental results demonstrated that the fractional impedance controller has superior contact force absorption performance compared with a conventional controller, especially for high-stiffness objects and high-velocity movement. This fractional impedance controller may be useful especially for the purpose of flexible contact for assistive and rehabilitation robots for people.     

瞬变速软体机器人研究综述

瞬变速软体机器人是一类能够在极短的时间内（不到1 s）,通过柔性材料大变形产生瞬时高速驱动效果（大于1倍身长/秒）的软体机器人。本文总结了瞬变速软体机器人的国内外发展现状,并按照不同驱动方式为其分类。梳理了瞬变速软体机器人的运动建模与仿真方法,并提出了一种多场耦合的仿真模式,讨论了仿真研究的技术特点、难点、准确性以及发展方向。总结了当前瞬变速软体机器人的应用,并展望其在未来不同场景下的设计和应用。     

A new design methodology of electrostrictive actuators for bio-inspired robotics

At present advanced robotics concepts require new and more suitable components to be exploited, especially in the fields of the biomimetic and soft robotics. In this sense, the actuation system represents one of the most limiting factors for the realization of robots with bio-inspired features and performances.This paper presents a new soft actuators based on electroactive polymers (EAPs) technology. The actuator is composed of a pre-stretched silicone film sputtered with a very thin gold film on both sides, working as electrodes. A particular folded geometry, implemented through an innovative fabrication process, allows to exploit the electrostrictive effect and to develop soft actuators suitable in many applications where softness and flexibility are necessary. The manufactured prototypes were developed on the basis of a parametric model that takes into account all geometric parameters and material characteristics. The proposed model is useful to estimate the performances of the actuator and to improve them.     

交互式机器人建模与仿真系统

研究并实现交互式机器人建模与仿真系统iRMSS(interactive robotics modeling & simulation system),为机器人仿真提供了精确实时的实验平台.iRMSS 包含两个子系统:一是3D 机器人建模子系统iRMS(interactive robotics modeling sub-system),完成机器人连杆和物理模型建模;二是基于MSRS(microsoft robotics studio)的交互式仿真子系统iRSS(interactive robotics simulation sub-system),完成实时的机器人动力学仿真并获取仿真动力学参数.iRMSS 充分发挥MSRS 集成一体化、精确度高、通用性强的优势,同时解决MSRS 在机器人建模和物理参数获取两方面存在的问题,实验结果证实了iRMSS 系统的可行性.     

Viscoelastic studies of human subscapularis tendon: relaxation test and a Wiechert model

Numerical techniques such as the finite element method employ the material constitutive laws for their analysis. With regards to finite element analysis involving viscoelastic solids, the Generalized Standard Linear Solid (Wiechert) model has been a popular choice among available constitutive laws. Although numerous models have been developed to specifically describe the viscoelastic behavior of tendons and ligaments, most of them have not been implemented in commercial finite element packages. This paper describes a stress relaxation test on the human subscapularis tendon, and then presents an approach for obtaining constitutive parameters of a Wiechert model for the human subscapularis tendon using experimental data from the aforementioned relaxation test. The approach is general and thus, can be applied to other tendons and ligaments, as well as any linear viscoelastic solid materials. The Wiechert model is required if finite element analysis using the commercial finite element package ANSYS is to be performed for a biomechanic structure composed of tendons and/or ligaments.     

Cooperative Mobile Robotics: Antecedents and Directions

There has been increased research interest in systems composed of multiple autonomous mobile robots exhibiting cooperative behavior. Groups of mobile robots are constructed, with an aim to studying such issues as group architecture, resource conflict, origin of cooperation, learning, and geometric problems. As yet, few applications of cooperative robotics have been reported, and supporting theory is still in its formative stages. In this paper, we give a critical survey of existing works and discuss open problems in this field, emphasizing the various theoretical issues that arise in the study of cooperative robotics. We describe the intellectual heritages that have guided early research, as well as possible additions to the set of existing motivations.     

Artificial neural network based robot control: An overview

The current thrust of research in robotics is to build robots which can operate in dynamic and/or partially known environments. The ability of learning endows the robot with a form of autonomous intelligence to handle such situations. This paper focuses on the intersection of the fields of robot control and learning methods as represented by artificial neural networks. An in-depth overview of the application of neural networks to the problem of robot control is presented. Some typical neural network architectures are discussed first. The important issues involved in the study of robotics are then highlighted. This paper concentrates on the neural network applications to the motion control of robots involved in both non-contact and contact tasks. The current state of research in this area is surveyed and the strengths and weakness of the present approaches are emphasized. The paper concludes by indentifying areas which need future research work.     

Prediction of Human Behavior in Human--Robot Interaction Using Psychological Scales for Anxiety and Negative Attitudes Toward Robots

When people interact with communication robots in daily life, their attitudes and emotions toward the robots affect their behavior. From the perspective of robotics design, we need to investigate the influences of these attitudes and emotions on human-robot interaction. This paper reports our empirical study on the relationships between people's attitudes and emotions, and their behavior toward a robot. In particular, we focused on negative attitudes, anxiety, and communication avoidance behavior, which have important implications for robotics design. For this purpose, we used two psychological scales that we had developed: negative attitudes toward robots scale (NARS) and robot anxiety scale (RAS). In the experiment, subjects and a humanoid robot are engaged in simple interactions including scenes of meeting, greeting, self-disclosure, and physical contact. Experimental results indicated that there is a relationship between negative attitudes and emotions, and communication avoidance behavior. A gender effect was also suggested.     

A note on aging of a viscoelastic cylinder

Rajagopal and Wineman [K.R. Rajagopal, A.S. Wineman, A note on viscoelastic materials that can age, Internat. J. Non-Linear Mech. 39 (2004) 1554–1574] showed that the stress relaxation due to viscoelastic effects and aging are distinctly different, in that the stress relaxation due to aging is geometry dependent. Here, I include the effect of stress relaxation in a Kelvin–Voigt solid, that cannot stress relax due to viscoelastic effects, due to aging. I also show that the viscoelastic contribution of the total torque and the normal force is such that space and time are separable, while the same cannot be said about the contribution due to aging.     

A simple computational homogenization method for structures made of linear heterogeneous viscoelastic materials

In this paper, a numerical multiscale method is proposed for computing the response of structures made of linearly non-aging viscoelastic and highly heterogeneous materials. In contrast with most of the approaches reported in the literature, the present one operates directly in the time domain and avoids both defining macroscopic internal variables and concurrent computations at micro and macro scales. The macroscopic constitutive law takes the form of a convolution integral containing an effective relaxation tensor. To numerically identify this tensor, a representative volume element (RVE) for the microstructure is first chosen. Relaxation tests are then numerically performed on the RVE. Correspondingly, the components of the effective relaxation tensor are determined and stored for different snapshots in time. At the macroscopic scale, a continuous representation of the effective relaxation tensor is obtained in the time domain by interpolating the data with the help of spline functions. The convolution integral characterizing the time-dependent macroscopic stress–strain relation is evaluated numerically. Arbitrary local linear viscoelastic laws and microstructure morphologies can be dealt with. Implicit algorithms are provided to compute the time-dependent response of a structure at the macroscopic scale by the finite element method. Accuracy and efficiency of the proposed approach are demonstrated through 2D and 3D numerical examples and applied to estimate the creep of structures made of concrete.     

Nonlinear viscoelastic contact and deformation of freeform virtual surfaces

This article is concerned with the haptic deformation display of discrete viscoelastic surfaces by means of a human fingertip. The virtual surface of a deformable quadrilateral mesh is interactively deformed by a Kelvin–Voigt soft fingertip model attached to the end-effector of a haptic interface device. In achieving this task, a nonlinear constitutive model approximating experimental data from literature is developed for determining the contact point deformations. By employing a new kernel weighting function, the deformations are distributed dependently on the discrete surface topology based on a nonlinear spring–damper net around the contact location. For illustration and evaluation of the proposed approach, a parallel robotic device with a constraint-based controller is adopted. The grip of the device is moved by the user to feel a sense of touch as the soft fingertip deforms the mesh surface of an ex vivo porcine liver tissue. Experimental data indicates stable realistic interactions thorough mechanical coupling between the soft fingertip and the deforming liver tissue. Dynamic response data of liver show rate-dependent hysteretic deformations and match closely with experimental indentation data from literature. A thorough analysis of mesh node count on the sample rate and the rendering quality is also presented.     

内凹面触觉传感器设计与接触过程分析

设计一种用于微创手术（ MIS）的圆锥形内凹面触觉传感器,不仅可以分析接触力,辨别不同软组织的相对硬度,而且能得到软组织的材料参数,进而辨识组织和可能的组织病变。首先,建立传感器与弹性体间的接触模型;然后,根据弹性与粘弹性对应原理建立传感器与粘弹性体的接触模型,解算出剪切模量等参数信息;最后,通过实验验证了该传感器的可行性。