Eric Chatelet

Contact of a Finger on Rigid Surfaces and Textiles: Friction Coefficient and Induced Vibrations

The tactile information about object surfaces is obtained through perceived contact stresses and friction-induced vibrations generated by the relative motion between the fingertip and the touched object. The friction forces affect the skin stress-state distribution during surface scanning, while the sliding contact generates vibrations that propagate in the finger skin activating the receptors (mechanoreceptors) and allowing the brain to identify objects and perceive information about their properties. In this article, the friction coefficient between a real human finger and both rigid surfaces and fabrics is retrieved as a function of the contact parameters (load and scanning speed). Then, the analysis of the vibration spectra is carried out to investigate the features of the induced vibrations, measured on the fingernail, as a function of surface textures and contact parameters. While the friction coefficient measurements on rigid surfaces agree with empirical laws found in literature, the behaviour of the friction coefficient when touching a fabric is more complex, and is mainly the function of the textile constructional properties. Results show that frequency spectrum distribution, when touching a rigid surface, is mainly determined by the relative geometry of the two contact surfaces and by the contact parameters. On the contrary, when scanning a fabric, the structure and the deformation of the textile itself largely affect the spectrum of the induced vibration. Finally, some major features of the measured vibrations (frequency distribution and amplitude) are found to be representative of tactile perception compared to psychophysical and neurophysiologic works in literature.

Experimental analysis of friction-induced vibrations at the finger contact surface:

Abstract When a finger moves to scan the surface of an object, the sliding contact generates vibrations that propagate in the finger skin and transmit the information about the object characteristics to mechanoreceptors. Mechanoreceptors convert vibrations into electric impulses sent to the brain. In this context, by appropriate experiments, a frequency analysis of the signal characterizing the surface scanning can be carried out to investigate the vibration spectrum measured on the finger and to highlight how the tactile sense is connected to the measured frequency spectra. Although the correlation between the surface roughness with respect to the tactile sensation is deeply analysed in the literature, the vibration spectra induced by the finger—surface scanning and the consequent activation of mechanoreceptors on the skin were rarely investigated. In particular, in this paper, interests will be focused on the changes shown in the vibration spectra, caused by variations in the characteristic contact parameters such as scanning velocity and roughness.

Rotating Internal Damping in the Case of Composite Shafts

There is an increasing range of applications for rotors made of composite materials and operating at supercritical speeds. Design of such structures involves specific features which have to be accounted for in order to allow safe operations. A proper modeling of the mechanical characteristics of the composite is first needed. But, as far as the structure is rotating, the effect of stress stiffening and spin softening may be considered and the effect of internal damping has to be studied in order to avoid possible instability. Internal or rotating damping modeling remains an active field of research where both theoretical developments and experimental results are needed.

Design of a tribometer for investigating tactile perception

The understanding of the tactile perception mechanism implies the reproduction and measurement of friction forces and vibrations induced by the contact between the skin of human fingers and object surfaces. When a finger moves to scan the surface of an object, it activates the receptors located under the skin allowing the brain to identify surfaces and information about their properties. The information concerning the object surface is affected by the forces and vibrations induced by the friction between the skin and the rubbed object. The vibrations propagate in the finger skin and are converted into electric impulses sent to the brain by the mechanoreceptors. Because of the low amplitude of the induced vibrations, it results quite hard to reproduce the tactile surface scanning and measuring it without affecting measurements by external noise coming from the experimental test-bench. In fact the reproduction of the sliding contact between two surfaces implies the relative motion between them, which is obtained by appropriate mechanisms having a more or less complicated kinematics and including several sliding surfaces (bearings, sliders, etc.). It results quite difficult to distinguish between the vibrations coming from the reproduced sliding and the parasitic noise coming from the other sliding contact pairs. This paper presents the design and validation of a tribometer, named TRIBOTOUCH, allowing for reproducing and measuring friction forces and friction induced vibrations that are basilar for a clear understanding of the mechanisms of the tactile sense.

Dynamic analysis of surface scanning for tactile perception

The human hand works in a perfect accord with the brain for an efficient exploration of physical world and objects perception according to man’s purposes. During the haptic sensing, the fingertip slides on a surface activating the receptors located under the skin allowing the brain to identify objects and information about their properties. In fact, in order to create the contact, the hand must exercise a force causing the fingertip to deform, generating a stress-state that contains the information on the object in contact. The information concerning the object surface is represented by the vibrations induced by the friction between the skin and the rubbed object in contact. The mechanoreceptors have the key role of transducing the stress state into an electrical impulse conveyed to the brain. Nevertheless, the vibration spectra induced by the finger/surface rubbing and the consequent activation of the mechanoreceptors on the skin were rarely investigated. A clear understanding of the mechanisms of the tactile sense is basilar for manifold applications, like the development of artificial tactile sensors for intelligent prostheses or robotic assistants, and for the ergonomics. In this context, it is fundamental to realize appropriate dynamic analysis of the signals that characterize the characteristics of the contact. In other words, it is necessary to investigate the vibration spectrum measured on the finger, in order to identify the frequency range of measured spectra (that should correspond to the expected one given by the mechanoreceptors activation frequency range [2–500 Hertz]). An experimental set-up is developed to recover the contact global dynamics by detecting the contact force and the induced vibrations; the bench test has been designed to guarantee the measurements reproducibility and, at cause of the low amplitude of the vibrations of interest, to perform measurements without introducing external noise. In particular, in this paper, the interest will be focused on the changes shown in vibration spectra with respect to variations of the scanning velocity and surface roughness characteristics.Copyright

Identification of Contact Area From Full Field Displacement Surface Measurements

One of the most common failure modes for turbomachinery wheels is associated to high-cycle fatigue of blades. A practical way to extend working life is obtained through the introduction of specific devices that allow a reduction in vibrational magnitudes during resonance. Different kinds of components are used such as shrouds and wires for power industries and under platform dampers for aeronautics. The dry friction phenomenon between those devices and the blades induces nonlinear behaviors and flatten associated frequency response functions. This phenomenon is now well known and different modeling techniques of contact are available within numerical simulation softwares. Nevertheless, it is always difficult to estimate or to measure with sufficient precision the actual contact characteristics needed to run those softwares. Due to practical experimental capabilities, measurements are only possible quite far from the contact zone and major quantities such as the transverse loading for example are often unreachable directly. In this paper, a new inverse methodology is presented. This method uses surface displacement measurements (obtained usually experimentally from conventional accelerometer and fast camera) in order to identify the characteristics of contact zones within elastic body assemblies. The new methodology is validated and illustrated by a numerical approach based on an academic set up

Passive defect detection in plate from nonlinear conversion of low-frequency vibrational noise

It is known that, under the assumption of diffuse noise, the cross-correlation of acoustic signals recorded at two points of a medium allows to passively estimate the impulse response between these points. This principle, associated with coherent array processing, has been successfully applied to defect detection and localization in reverberant plates subject either to distributed noise sources or friction noise. In order to extend the applicability of this principle even in the absence of an adequate ambient noise, we have introduced the concept of secondary noise sources based on the conversion of low-frequency modal vibrations into high-frequency noise by exploiting frictional contact nonlinearities. The device consists of a mass-spring resonator coupled to a flexible beam by a rough frictional interface. The extremity of the beam, attached to the surface of a plate, excites efficiently flexural waves in the plate in the ultrasound range when the primary resonator vibrates around its natural frequency....

DAMPING INDUCED BY DRY FRICTION: ANALYSES AND EXPERIMENTS FOR MODELING IMPROVEMENT

Abstract. The present study deals with the effects of contact characteristics on the vibratory response of a beam associated with several rubbing devices. Experimentally different contact configurations are tested by varying the contact area size and roughness of samples. For each tests, the frequency response functions (FRF) are measured and compared. Numerically, temporal calculations are presented and compared to measured data, using Masing’s contact model with updated parameter values and averaged estimated values.

Effect of contact parameter uncertainties of the dynamic behaviour of rubbing cyclic assemblies

Fatigue with high number of cycles (HCF) is a current and dangerous mode of failure for blades of turbo shaft engines. It is induced by the high dynamic stresses generated at resonance in the operating range. Rubbing devices based on the use of dry friction, such as shrouds or under-platform dampers, make it possible to reduce vibratory amplitudes and even to push back resonances out of the operation zone. The governing parameters of rubbing contacts are often not known with accuracy and then are not easily controlled. They may also be the source of potentially high level responses of blades due to mistuning effects. To illustrate the influence of uncertainties, a stochastic model has been developed allowing the study of the dynamic behaviour of bladed assemblies induced by random dry friction damper parameters. These parameters are expanded on the Karhunen-Loeve base with random variables following the Gaussian law. The random nonlinear problem is based on the Monte Carlo simulation and gives the statistical characteristics of the forced response in the case of a lumped cyclic system. Results obtained confirm and illustrate the effects of variable contact parameters on the dynamic behaviour of nonlinear rubbing systems.