Surface strain measurements of fingertip skin under shearing

The temporal evolution of surface strain, resulting from a combination of normal and tangential loading forces on the fingerpad, was calculated from high-resolution images. A customized robotic device loaded the fingertip with varying normal force, tangential direction and tangential speed. We observed strain waves that propagated from the periphery to the centre of the contact area. Consequently, different regions of the contact area were subject to varying degrees of compression, stretch and shear. The spatial distribution of both the strains and the strain energy densities depended on the stimulus direction. Additionally, the strains varied with the normal force level and were substantial, e.g. peak strains of 50% with a normal force of 5 N, i.e. at force levels well within the range of common dexterous manipulation tasks. While these observations were consistent with some theoretical predictions from contact mechanics, we also observed substantial deviations as expected given the complex geometry and mechanics of fingertips. Specifically, from in-depth analyses, we conclude that some of these deviations depend on local fingerprint patterns. Our data provide useful information for models of tactile afferent responses and background for the design of novel haptic interfaces.     

Effect of skin hydration on the dynamics of fingertip gripping contact

The dynamics of fingertip contact manifest themselves in the complex skin movements observed during the transition from a stuck state to a fully developed slip. While investigating this transition, we found that it depended on skin hydration. To quantify this dependency, we asked subjects to slide their index fingertip on a glass surface while keeping the normal component of the interaction force constant with the help of visual feedback. Skin deformation inside the contact region was imaged with an optical apparatus that allowed us to quantify the relative sizes of the slipping and sticking regions. The ratio of the stuck skin area to the total contact area decreased linearly from 1 to 0 when the tangential force component increased from 0 to a maximum. The slope of this relationship was inversely correlated to the normal force component. The skin hydration level dramatically affected the dynamics of the contact encapsulated in the course of evolution from sticking to slipping. The specific effect was to reduce the tendency of a contact to slip, regardless of the variations of the coefficient of friction. Since grips were more unstable under dry skin conditions, our results suggest that the nervous system responds to dry skin by exaggerated grip forces that cannot be simply explained by a change in the coefficient of friction.     

How to identify water from thickener aqueous solutions by touch

Water detection is one of the most crucial psychological processes for many animals. However, nobody knows the perception mechanism of water through our tactile sense. In the present study, we found that a characteristic frictional stimulus with large acceleration is one of the cues to differentiate water from water contaminated with thickener. When subjects applied small amounts of water to a glass plate, strong stick-slip phenomena with a friction force of 0.46 ± 0.30 N and a vertical force of 0.57 ± 0.36 N were observed at the skin surface, as shown in previous studies. Surprisingly, periodic shears with acceleration seven times greater than gravitational acceleration occurred during the application process. Finite-element analyses predicted that these strong stimuli could activate tactile receptors: Meissner''s corpuscle and Pacinians. When such stimuli were applied to the fingertips by an ultrasonic vibrator, a water-like tactile texture was perceived by some subjects, even though no liquid was present between the fingertip and the vibrator surface. These findings could potentially be applied in the following areas: materials science, information technology, medical treatment and entertainment.     

The geometry of vacuum assisted interface adhesion: Resistance to axial moment

Many engineering applications require joining two surfaces by temporary methods that do not harm either surface. The need for non-destructive surface bonding includes the need to resist axial moments. Suction cups in two different arrangements, in pairs at 180° apart and in fours at 90° apart, were pressed onto a polymerized thermoset polyester surface and then subjected to an axial moment. Defining failure as either sliding or complete loss of adhesion, the maximum torque applied before adhesive failure at the cup-plate interface was quantified using an MTS materials testing system. A linear relationship existed between the distance from the centroid of the cups and the applied torque at failure; also a linear relationship existed between the contact area of the cups and the tangential reaction force. Experiment verified that the applied load is distributed among the cups such that the tangential reaction force on each cup is proportional to its distance from the centroid; a coefficient of static friction between polyvinylchloride and polymerized thermoset polyester of 1.24 was calculated from the data.     

Influence of contact configuration on fretting fatigue behavior of Ti–6Al–4V under independent pad displacement condition

Fretting fatigue tests were conducted, using cylindrical pad and flat pad with rounded edges, at various applied pad displacements and at two normal forces on the pad under a constant bulk stress amplitude condition. The evolution of tangential force was independent of the contact configuration at a given normal force. The ratio of the tangential force to normal force increased and stabilized to a certain value with increasing applied pad displacement. The minimum fretting fatigue life was observed at the relative slip range between 50 and 60 μm and it was independent of both contact configuration and applied normal force. With increase in the applied pad displacement the response of the tangential force (Q) and the relative slip (δ) showed different fretting conditions, i.e. stick, stick-slip and gross slip. The gross slip condition was characterized by rectangular shape of the Q–δ curve with or without monotonically increasing value of Q with increasing fretting fatigue cycles. Surface profile on the fretting scar was affected by the contact configurations. For cylinder-on-flat contact, the profile showed surface damage (e.g. material loss or wear) along the entire contact area. However, the fretting damage in flat-on-flat (with rounded edges) contact was concentrated on the edge, not affecting much of the flat portion of the fretting scar.     

Tactile sensor based on piezoelectric resonance

We discuss here the realization of tactile sensors based on the principle of change in piezoelectric resonance frequency with the applied pressure. An array of electrodes has been adopted on either side of the PZT material to have independent resonators. The common areas sandwiched between the electrodes and excitable at resonance frequency of the PZT material are used to form the sensitive area of the tactile sensor. The electrodes were deposited using sputtering technique. Tactile sensors with 3/spl times/3, 7/spl times/7, and 15/spl times/15 array of electrodes are developed with different electrode dimensions and separation between the electrodes. The tactile sensor has been interfaced to computer for the convenience of automatic scanning and making it more user interactive. The tactile sensors developed with different spatial resolution were tested for different shaped objects placed in contact with the sensor. The 3/spl times/3 matrix tactile sensor showed relatively poor spatial resolution, whereas the 15/spl times/15- matrix tactile sensor showed improved spatial resolution. The sensor with 7/spl times/7 matrix elements was tested for its sensitivity to different extents of applied force/pressure. The output response study carried out on the sensors indicated that these sensors can provide information not only about the extent of force/pressure applied on the object, but also the contour of the object which is in contact with the sensor.     

State-of-the-Art in Force and Tactile Sensing for Minimally Invasive Surgery

Haptic perception plays a very important role in surgery. It enables the surgeon to feel organic tissue hardness, measure tissue properties, evaluate anatomical structures, and allows him/her to commit appropriate force control actions for safe tissue manipulation. However, in minimally invasive surgery, the surgeon's ability of perceiving valuable haptic information through surgical instruments is severely impaired. Performing the surgery without such sensory information could lead to increase of tissue trauma and vital organic tissue damage. In order to restore the surgeon's perceptual capability, methods of force and tactile sensing have been applied with attempts to develop instruments that can be used to detect tissue contact forces and generate haptic feedback to the surgeon. This paper reviews the state-of-the-art in force and tactile sensing technologies applied in minimally invasive surgery. Several sensing strategies including displacement-based, current-based, pressure-based, resistive-based, capacitive-based, piezoelectric-based, vibration-based, and optical-based sensing are discussed.     

Analysis of the apparent friction of polymeric surfaces

The apparent friction coefficient is the ratio between the tangential force and the normal load applied to moving body in contact with the surface of a material. This coefficient includes a so-called “true local friction” at the interface and a “geometrical friction” which is the ploughing effect. The material underneath a moving tip may display various types of behaviour: elastic, elastic–plastic where elastic and plastic strain are present in the contact area, or fully plastic. As is usual in polymers, the material behaviour is time and temperature dependent and may exhibit strain hardening. A surface flow line model of a scratching tip which links the apparent friction to the local friction and contact geometry was recently proposed. An inverse analysis is used in the present work to estimate the local friction from the measured apparent friction and a knowledge of the contact area and tip shape. The polymer true friction coefficient displays temperature and sliding speed dependency, which may be attributed to the surface thermodynamics. It is shown that the local friction depends on the level of strain in the polymer at the contact interface.     

轮对运动状态对轮轨滚动接触应力的影响

分析计算了锥型踏面轮对沿轨道滚动接触时轮轨接触几何参数和不同运动状态下的轮轨之间的刚性蠕滑率。根据确定的轮轨接触几何参数和轮轨接触界面之间的蠕滑率,利用非Hertz滚动接触理论分析计算了锥型轮对和钢轨滚动接触斑作用力的分布。再利用弹性力学中Bossinesq-Cerruti力/位移计算公式并借助Gauss数值积分方法,确定了轮轨滚动接触时体内的弹性位移、应变和应力随轮对运动状态变化情况。数据结果为轮轨强度设计提供了重要的参考依据。     

汽车内饰品质感的感知模态研究

目的从用户感知模态出发,构建内饰品质感性意象评价模型。方法以情感设计为背景,提出了用户对汽车内饰的品质感是由直接知觉和认知意象共同构成;从感知觉特性出发,通过30名用户对6个汽车内饰皮质纹理样本质感的评价进行情感语义量化,形成皮质纹理意象感知尺度。结果从触觉单模态感知和视触觉双模态感知的角度,验证了用户不同感觉模态下对汽车内饰皮质纹理情感的感知差异。结论基于汽车内饰品质的情感设计,其核心在于对用户感知与情感之间客观联系的挖掘,以材料工艺所表现的产品造型感知质量成为内饰设计的必然趋势。     

基于增量耦合预测控制的风电叶片打磨机械臂末端颤振抑制研究

针对风电叶片打磨过程中,末端执行器与叶片表面发生刚性接触而引起末端执行器切向颤振问题,提出一种基于力反馈与加速度前馈复合结构的末端执行器增量耦合预测控制方法。在末端执行器柔性驱动单元数学建模的基础上,基于增量耦合动态矩阵预测控制算法对复合PID控制策略进行改进,将不可控但可预知输入加速度作为磨削轴向力预测序列的一部分。同时,在有限时域内采用二次型性能指标最小化的方式对控制目标进行滚动优化,以确保末端执行器与风电叶片表面柔顺接触。仿真及实验结果表明,此方法可以快速地实现末端执行器的切向颤振抑制,并能最大限度地减小因控制时滞、环境时变、模型失配等带来的误差。     

Instability of the stressed surface of a highly viscous liquid

A new type of hydrodynamic instability is observed in a highly viscous liquid whose free surface is exposed to an external force acting at a certain angle to the normal. The physical reason for this instability is the redistribution of energy between the tangential and normal components of the free surface stresses. Pis’ma Zh. Tekh. Fiz. 25, 1–6 (November 26, 1999)     

Statistical properties of a 2D granular material subjected to cyclic shear

This work focuses on the evolution of structure and stress for an experimental system of 2D photoelastic particles that is subjected to multiple cycles of pure shear. Throughout this process, we determine the contact network and the contact forces using particle tracking and photoelastic techniques. These data yield the fabric and stress tensors and the distributions of contact forces in the normal and tangential directions. We then find that there is, to a reasonable approximation, a functional relation between the system pressure, P, and the mean contact number, Z. This relationship applies to the shear stress τ, except for the strains in the immediate vicinity of the contact network reversal. By contrast, quantities such as P, τ and Z are strongly hysteretic functions of the strain, ε. We find that the distributions of normal and tangential forces, when expressed in terms of the appropriate means, are essentially independent of strain. We close by analyzing a subset of shear data in terms of strong and weak force networks.     

A Dual-Responsive Artificial Skin for Tactile and Touchless Interfaces

The progress from intelligent interactions and supplemented/augmented reality requires artificial skins to shift from the single-functional tactile paradigm. Dual-responsive sensors that can both detect pre-contact proximal events and tactile pressure levels enrich the perception dimensions and deliver additional cognitive information. Previous dual-responsive sensors show very limited utilizations only in proximity perception or approaching switches. Whereas, the approaching inputs from the environment should be able to convey more valuable messages. Herein, a flexible iontronic dual-responsive artificial skin is present. The artificial skin is sensitive to external object's applied pressure as well as its approaching, and can elicit information of target material categories encoded in the proximal inputs. Versatile applications are then demonstrated. Dual-mode human–machine interfaces are developed based on the devices, including a manipulation of virtual game characters, navigation and zooming in of electronic maps, and scrolling through electronic documents. More importantly, the proof-of-concept application of an entirely touchless material classification system is demonstrated. Three types of materials (metals, polymers, and human skins) are classified and predicted accurately. These features of the artificial skin make it highly promising for next-generation smart engineered electronics.     

Influence of epidermal hydration on the friction of human skin against textiles

Friction and shear forces, as well as moisture between the human skin and textiles are critical factors in the formation of skin injuries such as blisters, abrasions and decubitus. This study investigated how epidermal hydration affects the friction between skin and textiles.The friction between the inner forearm and a hospital fabric was measured in the natural skin condition and in different hydration states using a force plate. Eleven males and eleven females rubbed their forearm against the textile on the force plate using defined normal loads and friction movements. Skin hydration and viscoelasticity were assessed by corneometry and the suction chamber method, respectively.In each individual, a highly positive linear correlation was found between skin moisture and friction coefficient (COF). No correlation was observed between moisture and elasticity, as well as between elasticity and friction. Skin viscoelasticity was comparable for women and men. The friction of female skin showed significantly higher moisture sensitivity. COFs increased typically by 43% (women) and 26% (men) when skin hydration varied between very dry and normally moist skin. The COFs between skin and completely wet fabric were more than twofold higher than the values for natural skin rubbed on a dry textile surface.Increasing skin hydration seems to cause gender-specific changes in the mechanical properties and/or surface topography of human skin, leading to skin softening and increased real contact area and adhesion.     

回转窑滚圈与托轮接触应力解析

 滚圈与托轮接触应力的计算对回转窑滚动接触疲劳寿命预测起到至关重要的作用。考虑滚圈与托轮在牵引滚动接触状态下相互之间传递法向力和切向摩擦力的综合作用,将其简化为二维平面应变问题,然后从经典Hertz理论中弹性半空间在分布法向力和切向力作用下受载区应力分量的积分表达式出发,对接触区对称平面上各点的应力进行分析与计算,得出受载区各主应力的解析表达式,并采取最优函数逼近法求得半解析解,确定了最大主剪应力及其位置,能满足工程应用的要求。     

Study of friction and wear mechanisms at high sliding speed

The aim of this work is to propose an analysis of mechanisms inducing surface interaction by friction during high sliding speed. Specific devices including a ballistic setup were used to reproduce extreme sliding conditions combining high speed and high pressure. The titanium alloy/tantalum tribo-pair is chosen to investigate the frictional and material transfer mechanisms. The tangential force measurement is used to follow the evolution of the friction coefficient at a macroscopic scale. The evolution of the sliding surface was analyzed by confocal 3D microscope to evaluate material transfer and real contact surface area. Numerical modeling of micro-contact at the asperities scale is presented to illustrate the scenarii involved during friction. The energy needed to shear a junction is estimated and analyzed for several types of interaction. Different behaviors have been taken into account in order to investigate the global forces generated by the contact including strong and weak contacts. The analysis of energy is available to predict the global friction force in a large range of velocities. Correlations between experimental measurements and numerical predictions are used to validate the proposed approach. The results can be interpreted as following: (1) at lower velocity the main mechanism dominating the interaction between asperities becomes ploughing with large volume of plastic deformation (2) at higher velocity the main mechanism is shear localization requiring less energy and force for shearing the junctions.     

Highly Morphology‐Controllable and Highly Sensitive Capacitive Tactile Sensor Based on Epidermis‐Dermis‐Inspired Interlocked Asymmetric‐Nanocone Arrays for Detection of Tiny Pressure

The tactile sensor lies at the heart of electronic skin and is of great importance in the development of flexible electronic devices. To date, it still remains a critical challenge to develop a large‐scale capacitive tactile sensor with high sensitivity and controllable morphology in an economical way. Inspired by the interlocked microridges between the epidermis and dermis, herein, a highly sensitive capacitive tactile sensor by creating interlocked asymmetric‐nanocones in poly(vinylidenefluoride‐co‐trifluoroethylene) film is proposed. Particularly, a facile method based on cone‐shaped nanoporous anodized aluminum oxide templates is proposed to cost‐effectively fabricate the highly ordered nanocones in a controllable manner and on a large scale. Finite‐element analysis reveals that under vertical forces, the strain/stress can be highly strengthened and localized at the contact apexes, resulting in an amplified variation of film permittivity and thickness. Benefiting from this, the developed tactile sensor presents several conspicuous features, including the maximum sensitivity (6.583 kPa^?1) in the low pressure region (0–100 Pa), ultralow detection limit (≈3 Pa), rapid response/recovery time (48/36 ms), excellent stability and reproducibility (10 000 cycles). These salient merits enable the sensor to be successfully applied in a variety of applications including sign language gesture detection, spatial pressure mapping, Braille recognition, and physiological signal monitoring.     

Spatio-temporal skin strain distributions evoke low variability spike responses in cuneate neurons

A common method to explore the somatosensory function of the brain is to relate skin stimuli to neurophysiological recordings. However, interaction with the skin involves complex mechanical effects. Variability in mechanically induced spike responses is likely to be due in part to mechanical variability of the transformation of stimuli into spiking patterns in the primary sensors located in the skin. This source of variability greatly hampers detailed investigations of the response of the brain to different types of mechanical stimuli. A novel stimulation technique designed to minimize the uncertainty in the strain distributions induced in the skin was applied to evoke responses in single neurons in the cat. We show that exposure to specific spatio-temporal stimuli induced highly reproducible spike responses in the cells of the cuneate nucleus, which represents the first stage of integration of peripheral inputs to the brain. Using precisely controlled spatio-temporal stimuli, we also show that cuneate neurons, as a whole, were selectively sensitive to the spatial and to the temporal aspects of the stimuli. We conclude that the present skin stimulation technique based on localized differential tractions greatly reduces response variability that is exogenous to the information processing of the brain and hence paves the way for substantially more detailed investigations of the brain''s somatosensory system.     

旋转型行波超声电机理论模型的仿真研究

建立了旋转型行波超声电机定、转子间的摩擦驱动模型，不仅考虑了接触界面上的纵向分布力，而且分析接触力沿周向和径向上的分量，在此基础上结合定子和转子的动力学模型得到了整个电机的机电耦合模型。文章分析了预压力影响电机驱动频率的本质原因，说明该频率不能直接通过分析自由定子得到，最后通过数值计算分析了接触界面力对电机工作频率的影响，取得了与实验一致的结果。