Normal Force Components Research Articles

Selective Effects of Arm Proximal and Distal Muscles Fatigue on Force Coordination in Manipulation Tasks

ABSTRACT. Effects of muscle fatigue on force coordination and task performance of various manipulation tasks are explored. Grip force (GF; normal force component acting at the digits-object contact area) and load force (LF; tangential component that lifts and holds objects) were recorded prior to and after fatiguing the distal (DAM; i.e., GF producing) and proximal arm muscles (PAM; LF producing). Results reveal a deterioration of GF scaling (i.e., averaged GF-LF ratio), GF-LF coupling (their correlation), and task performance (ability to exert a prescribed LF pattern) associated with DAM, but not PAM fatigue. Deteriorated force coordination clearly increases the likelihood of dropping an object; however, the observed selective effects of DAM and PAM fatigue represent a novel finding deserving of further research.

An artificial skin based on optoelectronic technology

This work proposes an artificial skin composed by independent sensing modules, able to estimate both normal and shear contact force components. Each sensing module consists of four couples (taxels) of infrared Light Emitting Diodes and a Photo-Detectors, covered by a silicone layer that transduces the external force in a mechanical deformation. The applied force vector is estimated as a suitable combination of the four voltage signals measured by the four receivers. A complete prototype, with a 6×6 matrix of sensing modules, has been realized, characterized and tested. The prototype is able to discriminate multiple contact areas and to estimate the force resultants for each contact area. Envisaged applications are safe physical human–robot interaction, where contact forces over large distributed areas can occur. The use of a dedicated distributed sensor for measuring the contact force vectors is aimed at overcoming current methods based on the use of dynamic models and joint torque measurements.

Internal Forces During Static Prehension: Effects of Age and Grasp Configuration

ABSTRACTThe authors studied effects of healthy aging on 3 components of the internal force vector during static prehensile tasks. Young and older subjects held an instrumented handle using a 5-digit prismatic grasp under different digit configurations and external torques. Across digit configurations, older subjects showed larger internal normal (grip) and tangential (load-resisting) digit force components and larger internal moment of force. In contrast to earlier reports, safety margin values were not higher in the older subjects. The results show that the increased grip force in older persons is a specific example of a more general age-related problem reflected in the generation of large internal force vectors in prehensile tasks. It is possible that the higher internal forces increase the apparent stiffness of the hand+handle system and, hence, contribute to its stability. This strategy, however, may be maladaptive, energetically wasteful, and inefficient in ensuring safety of hand-held objects.

An Optoelectronic Artificial Skin for Contact Force Vector Estimation

This work proposes a novel artificial skin, developed by using the optoelectronic technology, able to estimate both normal and shear contact force components. Each sensing module is obtained with four taxels, each one constituted by an infrared Light Emitting Diode and a Photo-Detector, covered by a silicone layer that transduces the external force in a mechanical deformation, measured by the four receivers. The applied force is estimated as a suitable combination of the four voltage signals. The distributed sensor is realized by interconnecting several sensing modules with a suitably designed scanning strategy. A complete prototype, with a 6 × 6 matrix of sensing modules has been realized, calibrated and tested.

Semi-analytical Force Model for Grinding Operations

Grinding process is generally used to improve the tolerance integrity and surface quality of a workpiece. However, in case of some hard-to-machine materials grinding can also be a cost effective alternative even for roughing operations. It is crucial to know process forces since they are necessary to identify the conditions for surface burn which is one of the most important issues in grinding applications. In this paper, a new semi-analytical force model for grinding process is developed by modeling abrasive grits and their interaction with the workpiece individually. Grits are examined to determine their geometrical properties and distribution on the grinding wheel. Semi-analytical equations for total normal and tangential force components as well as average force per grit are established by using the micro milling analogy. Fundamental parameters such as shear stress and friction coefficient between the grits and the work material are identified. The model can then be used in prediction of the forces for different cases involving the same material and the abrasive grain however with different conditions. The model predictions are verified by several experiments and also using Johnson-Cook material model.

Physical Factors Influencing Pleasant Touch during Tactile Exploration

BackgroundWhen scanning surfaces, humans perceive some of their physical attributes. These percepts are frequently accompanied by a sensation of (un)pleasantness. We therefore hypothesized that aspects of the mechanical activity induced by scanning surfaces with fingertips could be objectively associated with a pleasantness sensation. Previously, we developed a unidimensional measure of pleasantness, the Pleasant Touch Scale, quantifying the pleasantness level of 37 different materials. Findings of this study suggested that the sensation of pleasantness was influenced by the average magnitude of the frictional forces brought about by sliding the finger on the surface, and by the surface topography. In the present study, we correlated (i) characteristics of the fluctuations of frictional forces resulting from the interaction between the finger and the surface asperities as well as (ii) the average friction with the sensation of pleasantness.ResultsEight blindfolded participants tactually explored twelve materials of the Pleasant Touch Scale through lateral sliding movements of their index fingertip. During exploration, the normal and tangential interaction force components, fN and fT, as well as the fingertip trajectory were measured. The effect of the frictional force on pleasantness sensation was investigated through the analysis of the ratio fT to fN, i.e. the net coefficient of kinetic friction, μ. The influence of the surface topographies was investigated through analysis of rapid fT fluctuations in the spatial frequency domain. Results showed that high values of μ were anticorrelated with pleasantness. Furthermore, surfaces associated with fluctuations of fT having higher amplitudes in the low frequency range than in the high one were judged to be less pleasant than the surfaces yielding evenly distributed amplitudes throughout the whole spatial frequency domain.ConclusionCharacteristics of the frictional force fluctuations and of the net friction taking place during scanning can reliably be correlated with the pleasantness sensation of surfaces.

Grip and load force coordination in cyclical isometric manipulation task is not affected by the feedback type

BackgroundThe relationship between normal and tangential force components (grip force – GF and load force – LF, respectively) acting on the digits-object interface during object manipulation reveals neural mechanisms involved in movement control. Here, we examined whether the feedback type provided to the participants during exertion of LF would influence GF-LF coordination and task performance.MethodsSixteen young (24.7 ±3.8 years-old) volunteers isometrically exerted continuously sinusoidal FZ (vertical component of LF) by pulling a fixed instrumented handle up and relaxing under two feedback conditions: targeting and tracking. In targeting condition, FZ exertion range was determined by horizontal lines representing the upper (10 N) and lower (1 N) targets, with frequency (0.77 or 1.53 Hz) dictated by a metronome. In tracking condition, a sinusoidal template set at similar frequencies and range was presented and should be superposed by the participants’ exerted FZ. Task performance was assessed by absolute errors at peaks (AEPeak) and valleys (AEValley) and GF-LF coordination by GF-LF ratios, maximum cross-correlation coefficients (rmax), and time lags.ResultsThe results revealed no effect of feedback and no feedback by frequency interaction on any variable. AEPeak and GF-LF ratio were higher and rmax lower at 1.53 Hz than at 0.77 Hz.ConclusionThese findings indicate that the type of feedback does not influence task performance and GF-LF coordination. Therefore, we recommend the use of tracking tasks when assessing GF-LF coordination during isometric LF exertion in externally fixed instrumented handles because they are easier to understand and provide additional indices (e.g., RMSE) of voluntary force control.

Control of Finger Force Vectors With Changes in Fingertip Referent Coordinates

ABSTRACT The central hypothesis explored in the experiment is that adjustments of fingertip force vectors during object manipulation could result from a simple scaling rule applied to commands to individual digits. The commands have been associated with referent coordinates of the digit tips. The subjects performed quick lifting movements (over 20 cm in under 0.5 s) of a horizontally oriented handle with different combinations of the external load and torque. The prismatic grasp was used with the 4 fingers pressing on the bottom of the handle and the thumb acting on its top. Principal component and correlation analyses applied to the normal and tangential force vector components confirmed that the force direction of each digit was kept nearly constant in the object-centered referent frame across the loading conditions and movement phases. The middle and ring fingers showed weaker correlations between the force components as compared to the index and little fingers. The differences were likely related to the different roles of the normal force components in the moment of force production. The neural control of the hand, within the studied task, may be adequately described as a simple rule applied to a handful of parameters, such as the referent digit-tip coordinates.

On Computing Reliable Optimal Grasping Forces

This paper presents algorithms for optimal grasping forces. The previous work reveals that contact forces with minimal sum or maximum of normal force components can be written as positive combinations of primitive contact forces, to which the corresponding primitive contact wrenches express the required resultant wrench as their positive combination with minimum coefficients in terms of the L1 or L∞ metric. On this basis, we first propose an algorithm to compute such a set of primitive contact forces and the minimum contact forces. Moreover, considering the uncertainty in the actual friction coefficient, we develop another algorithm to determine the minimum required friction coefficient and the corresponding minimum contact forces within a given limit on their magnitude so that such contact forces are more reliable in practice. As our algorithms do not rely on any general optimization technique, they are very efficient and easy to implement. These algorithms can also be used in grasp quality evaluation and optimal grasp planning.

Grasp evaluation and contact points planning for polyhedral objects using a ray-shooting algorithm

Grasp evaluation and planning are two fundamental issues in robotic grasping and dexterous manipulation. Most traditional methods for grasp quality evaluation suffer from non-uniformity of the wrench space and a dependence on the scale and choice of the reference frame. To overcome these weaknesses, we present a grasp evaluation method based on disturbance force rejection under the assumption that the normal component of each individual contact force is less than one. The evaluation criterion is solved using an enhanced ray-shooting algorithm in which the geometry of the grasp wrench space is read by the support mapping. This evaluation procedure is very fast due to the efficiency of the ray-shooting algorithm without linearization of the friction cones. Based on a necessary condition for grasp quality improvement, a heuristic searching algorithm for polyhedral object regrasp is also proposed. It starts from an initial force-closure unit grasp configuration and iteratively improves the grasp quality to find the locally optimum contact points. The efficiency and effectiveness of the proposed algorithms are illustrated by a number of numerical examples.

Analysis of Postbuckling Drillstring Vibrations in Rotary Drilling of Extended-Reach Wells

One of the most serious concerns of extended-reach drilling is the dynamic behavior of the drillstring and the cleaning of well. Good cleaning requires an increased angular velocity. This paper presents a 3D nonlinear dynamic model of drillstring in a wellbore of 3D profile. The model suggests possible contact/lift-off of drill pipes with/from the wellbore wall. The interaction of lateral, torsional, and axial vibrations is taken into account. The relation between the normal component of contact force and the deformation of the wellbore wall is taken as quadratic-elastic. The friction force is described based on a hysteretic dynamic model. The friction force model also takes into account, the transition from a sliding to whirling. The equations of drillstring dynamics are solved numerically using the method of lines. The DYNTUB software is developed to analyze the drillstring time-varying processes under different loads. The program is used to study the effects of angular velocity, compression load, torque, friction factor, well profile, and availability of connectors on the drillstring dynamic behavior. From the study follows the key conclusions: (1) The friction factor has a considerable effect on the drillstring rotational behavior in the wellbore; (2) no whirling of drillstring at real value of rolling friction factor in a horizontal well in the discussed examples could be seen at all; (3) when whirling takes place, the contact force shows a dramatic times increase; and (4) snaking can be seen in any wells at moderate compressive load and angular velocity.

Effects of surfactant on terminal velocity of a Taylor bubble in a vertical pipe

Effects of soluble surfactant on the terminal velocity of a Taylor bubble rising through a vertical pipe are investigated using an interface tracking method. A level set method is utilized to track the interface. Transport of surfactant in the bulk liquid and at the interface is taken into account. The amount of adsorption and desorption is evaluated using the Frumkin and Levich model. The normal component of surface tension force is computed using a ghost fluid method, whereas the tangential component, i.e., the Marangoni force, is evaluated by making use of the continuum surface force model. Simulations of small air bubbles contaminated with soluble surfactant are carried out for validation. The Marangoni effects on the bubbles, i.e., the surface immobilization and the increase in drag coefficient, are well predicted. Then Taylor bubbles rising through vertical pipes filled with contaminated water at a low Morton number are simulated for various Eötvös numbers, various bulk surfactant concentrations and two different surfactants, i.e., 1-pentanol and Triton X-100. As a result, the following conclusions are obtained: (1) the reduction of surface tension near the bubble nose is the cause of the increase in terminal velocity, (2) the surfactant does not affect the terminal velocities of high Eötvös number bubbles since the bubbles at high Eötvös numbers are independent of surface tension, (3) the terminal velocity of a low Morton number Taylor bubble can be evaluated by making use of available correlations for clean Taylor bubbles, provided that the degree of contamination near the bubble nose is known and the Marangoni effect in the nose region is negligible, and (4) the Hatta number, which is the ratio of the adsorption velocity to the bubble velocity, is a primary factor governing the degree of contamination in the bubble nose region.

Ultimate strength, ripples, sound velocities, and density of phonon states of strained graphene

We study the dispersion characteristics of strained graphene using many-body interatomic potentials and find: (i) borders of the structural stability of a flat graphene in the three-dimensional space of the strain components ( ε xx , ε yy , ε xy ); (ii) sound velocities of strained graphene; and (iii) phonon density of states (DOS) of strained graphene. The border of structural stability of flat graphene is also presented in the space of components of normal and shear membrane forces ( T x , T y , T xy ). We find that flat graphene is structurally stable under elastic strain up to 0.3–0.4, but it becomes unstable to a shear strain in the absence of tensile components of strain. Also graphene cannot remain flat under compressive membrane forces because its bending stiffness vanishes. We employ the molecular dynamics simulations to study the post-critical behavior of graphene. We demonstrate that ripples with controllable amplitude and orientation can be generated under simultaneous action of shear and tensile membrane forces. Gaps in the phonon DOS are observed when graphene is strained close to the appearance of ripples. Sound velocities of unstrained graphene do not depend on the propagation direction but application of strain makes graphene anisotropic. One of the sound velocities vanishes at the border of the structural stability of graphene meaning that vanishing of sound velocity (or corresponding elastic constant) predicts impending instability.

The Effects of Machining Parameters on the Position of Resultant Force in Sawing of Granite

The effects of the machining parameters on the relative position of the resultant force were analyzed in circular sawing of granite with a diamond segmented blade. The horizontal and vertical force components and the power were measured. Calculated tangential and normal force components were subsequently used to calculate the relative position of resultant force besides the horizontal and vertical force components. It was found that the value of the relative position of resultant force in down sawing is less than that in up sawing. The shift of sawing swarf has heavy influence on the relative position of resultant force.

Sensitivity tuning of A 3-axial piezoresistive force sensor

Design and development of a Si based full membrane piezoresistive 3D force sensor is presented in this paper. Four piezoresistors are formed by ion implantation on the back side of a thin Si membrane, while on the front side a 380µm high Si mesa is produced by subtractive dry etching (deep reactive ion etching). The external force is applied on the top of the mesa. The applied force vector, i.e. its normal and shear force components are determined by combining the responses of the four piezoresistors. The effect of different parameters - the shape and thickness of the membrane, the cross section of the mesa - on the sensitivity of the sensor is analyzed systematically by finite element methods. Comparison of the characteristics of the different sensor designs obtained from simulations and from experimental measurements is also presented.

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.

Simultaneous normal and torsional force measurement by cantilever surface contour analysis

This study presents an alternative to the current Hooke's law-based force relation between rectangular cantilever deflection and applied force. In the new approach, a transduction constant is presented that (1) includes no cross-talk between torsion and normal force components, (2) is independent of the load application point, and (3) does not depend on the cantilever beam length. Rather than measuring the cantilever deformation at a single point (such as the tip location), it is measured at multiple adjacent points using scanning white light interferometry to provide a three-dimensional description of the cantilever deformation during loading. This measurement, processed by a force relation based on a superposition of deflections derived from Euler–Bernoulli bending theory and St Venant's torsion theory, provides the vertical, axial, and torsional force components simultaneously. Experimental results are compared to force predictions for the vertical and torsional components using macro-scale cantilevers under mass loading. An uncertainty analysis is also provided.

Nanofluid Surface Wettability Through Asymptotic Contact Angle

This investigation introduces the asymptotic contact angle as a criterion to quantify the surface wettability of nanofluids and determines the variation of solid surface tensions with nanofluid concentration and nanoparticle size. The asymptotic contact angle, which is only a function of gas-liquid-solid physical properties, is independent of droplet size for ideal surfaces and can be obtained by equating the normal component of interfacial force on an axisymmetric droplet to that of a spherical droplet. The technique is illustrated for a series of bismuth telluride nanofluids where the variation of surface wettability is measured and evaluated by asymptotic contact angles as a function of nanoparticle size, concentration, and substrate material. It is found that the variation of nanofluid concentration, nanoparticle size, and substrate modifies both the gas-liquid and solid surface tensions, which consequently affects the force balance at the triple line, the contact angle, and surface wettability.

Orientation angle and the adhesion of single gecko setae

We investigated the effects of orientation angle on the adhesion of single gecko setae using dual-axis microelectromechanical systems force sensors to simultaneously detect normal and shear force components. Adhesion was highly sensitive to the pitch angle between the substrate and the seta's stalk. Maximum lateral adhesive force was observed with the stalk parallel to the substrate, and adhesion decreased smoothly with increasing pitch. The roll orientation angle only needed to be roughly correct with the spatular tuft of the seta oriented grossly towards the substrate for high adhesion. Also, detailed measurements were made to control for the effect of normal preload forces. Higher normal preload forces caused modest enhancement of the observed lateral adhesive force, provided that adequate contact was made between the seta and the substrate. These results should be useful in the design and manufacture of gecko-inspired synthetic adhesives with anisotropic properties, an area of substantial recent research efforts.

A GJK-based approach to contact force feasibility and distribution for multi-contact robots

This paper presents a new approach to two fundamental problems concerning the equilibrium of a multi-contact robot: the contact force feasibility (CFF) and the contact force distribution (CFD). The CFF is to determine if there exist feasible contact forces to maintain a robot in equilibrium without breaking its contacts with the environment, while the CFD is to compute the minimum contact forces if a feasible solution exists. A general measure of overall contact force magnitude is defined, which includes the traditional measure (i.e., the sum of normal force components) and a more complex measure (i.e., the maximum of normal force components) as special cases. We first reduce the two problems into verifying the existence of nonnegative solutions and determining the nonnegative minimum one-norm solution to a system of linear equations, respectively. To obtain the explicit formulation of the linear system, it is required to compute the Minkowski sum of point sets, which usually is computationally expensive. Then, based on the GJK distance algorithm, we develop an iterative algorithm, which enables us to solve the linear system without calculating the Minkowski sum and compute the CFF and CFD in real time.