Design and validation of an experimental set-up for the analysis of friction induced vibrations at the finger contact surface Ramona Fagiani1-2, Francesco Massi1, Eric Chatelet1, Yves Berthier1, Aldo Sestieri 2 1 Université de Lyon, CNRS, INSA-Lyon, LaMCoS UMR5259, F-69621, France E-mail:
[email protected],
[email protected],
[email protected],
[email protected] 2 Department of Mechanics and Aeronautics, University "La Sapienza" of Rome, Italy E-mail:
[email protected],
[email protected] Keywords: Tactile perception, friction induced vibrations, tribology.
SUMMARY. When a finger moves to scan the surface of an object, it activates the receptors located under the skin allowing the brain to identify objects and information about their properties. 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 vibrations propagate in the finger skin and are converted into electric impulses sent to the brain by the mechanoreceptors. Nevertheless, the relationship between the friction induced vibration and the tactile sensation is rarely investigated, while 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 necessary to perform appropriate experiments to find out the frequency characteristics of the vibrations induced by the surface scanning, looking for how the tactile sense is connected to the frequency spectra of the induced vibrations. The study of a finger that moves on a surface involves different difficulties that are related to the material characteristics and to the measurements themselves. In fact, the goal is the measurement and analysis of the vibrations induced by the scanning, which are very low in magnitude; thus, it is complicated to isolate them from the vibration noise coming out from the experimental set-up and to detect them without significant alteration. Moreover the impossibility of measuring and, at the same time, maintaining the fidelity of the contact between skin and object surface, implies the choice of analyzing first the "global dynamics" (vibrations) of the human finger. Then, measurements of the local dynamics can be performed on a fake finger and correlated with the global measurements on the real one. For these reasons, an experimental set-up named TRIBOTOUCH is developed to recover the contact forces and the induced vibrations. The test bench has been designed to guarantee the measurements reproducibility and, due to the low amplitude of the vibrations of interest, to perform measurements without introducing external noise. The relative motion between finger and surface is obtained without any other interface under sliding contact by a compliant system. As support to the set-up validation, the results from a first analysis of the right hand finger behavior showing the influence of the scanning speed and surface roughness on the vibration spectra are showed in the paper.
1 INTRODUCTION The sensory perception is obtained through an appropriate combination of the different senses, each one specialized in decoding specific information, supplying global information. Specifically,
in this paper, the attention is focused on the tactile perception because a clear understanding of the tactile sense is essential for many applications: ergonomics of everyday objects; textile quality; identification of surface imperfections; tactile sensitivity during diagnosis or monitoring in rehabilitation processes; design of tactile communication devices; development of artificial tactile sensors for intelligent prostheses or robotic assistants; development of human-machine interfaces; reproduction of real perception (virtual reality); increase of human perception (augmented reality). In a human hand, there are different types of afferent units, located under the skin, classified on the basis of their properties, in nociceptive units (detecting thermal and mechanical pain), thermoreceptive units (thermal information) and mechanoreceptive units (responding to mechanical excitations) [1,2]. Focusing our attention on the touch sensation, mechanoreceptors, activated by vibrations induced from the finger scanning, are central to tactile perception. Due to the mechanical loads generated by a contact, the fingertip skin surface is deformed. At any instant of time, during contact, there is a space-time variation of the stress-state that causes the mechanoreceptors to respond with an appropriate space-time variation of their discharge rate [3].
Figure 1. Schematic representation of the contact between finger and object surface and scheme of the mechanoreceptors position into the skin. The scanning direction is along the line of the finger (from left to right, with respect to the figure). The tactile units are made by an afferent fiber and its unmyelinated ending that distinguishes them as Merkel disk, Meissner's corpuscles, Ruffini endings and Pacinian corpuscles (Fig.1). These endings are associated with four different tactile afferents populations, respectively: the SA I (slowly adapting type I, react from about 2 Hz to 16 Hz transmitting to the brain a sensation of pressure and a spatial neural image of the spatial structure of the object surface), the FA I (fast adapting type I, are easily excited between 3 Hz and 40 Hz, and produce the sensation of flutter transmitting a neural image of skin motion to the brain), the FA II (fast adapting type II, very sensitive to mechanical transients and vibrations of higher frequencies, reacting in a range of 40500 Hz), and SA II (slowly adapting type II, react from 100 Hz to 500 Hz, inducing a buzzing
sensation. They contribute to the perception of motion direction, hand shape, finger position and contact force through the pattern of skin stretch). The relative motion between a surface and a finger under adequate applied force and scanning speed permits perception of the surface characteristics. The perception mechanism of roughness is based on the “duplex model of tactile roughness perception” [4, 5]. From this point of view the perception of fine textures (spatial period smaller than 100 µm), highlighted by vibrations, is different from the coarse ones (spatial period is larger than 200 µm), characterized by a “single spatial intensive code”, mediated by SA I afferents. This theory has been supported experimentally by neurophysiologic studies [3, 4], and recently validated by measurements obtained by the presented test bench [5]. In this context, it is necessary to perform appropriate experiments to find out the frequency characteristics of the vibrations induced by surface scanning, looking for how tactile sensing is related to the measured frequency spectra. In fact, while the friction aspects of the scanning fingersurface contact are quite well defined, (several studies on the effects of loads [6, 7], pressure, and finger inclination with respect to the surface, moisturizers [8-11], are reported in the literature), the induced vibration spectra and the consequent activation of the mechanoreceptors on the skin were rarely investigated. The analysis of vibration spectra induced by scanning the surface can contribute to the definition of objective indexes about the surface perception (softness, textile quality, perceived roughness, etc.). This paper describes the experimental set-up developed for a direct approach, investigating the induced vibration spectra. Particular attention is addressed to the maintenance of the real contact condition and to the isolation of the vibration of interest from parasitic noise coming from the test bench. 2 THE TRIBOTOUCH SET UP DESIGN The study of a finger that moves on a surface involves difficulties that are related to the material characteristics and to the measurements themselves. In fact, it is practically impossible to measure the vibrations directly at the contact region between the skin and the sample surface without modifying the contact and it is extremely difficult to isolate vibrations from the parasitic noise coming from the set-up, because of the low amplitude of the friction induced vibrations. To avoid errors due to the modification of the contact pair it has been decided to analyze, first, the global measurements of the system dynamics, by an accelerometer mounted on the fingernail of a real finger; then, measurements on a fake finger allow for the analysis at the contact interface. A compliant mechanisms and a linear voice coil actuator have been used for the design of the setup rather than the driving system of a classical set-up, in order to avoid any other sliding contact pair generating vibration noise. The designed experimental set-up (Fig.2), named TriboTouch, carries the sample surface (a) translated by a compliant mechanism (b), which allows linear motion of the sample without the involvement of any other sliding contact. Specifically, the compliant guide can be approximated by a rigid-body mechanism consisting of rigid links connected to each other by flexible joints (c). For system stability, two nominally identical compliant mechanisms are placed symmetrically with respect to the median plane and slightly tilted toward each other, as shown in Fig.2. Each element is a double parallelogram that allows for a linear translation of the upper rigid segment where the sample is mounted (a). The flexible joints (c) approximate pin joints between the segments of the double parallelograms and are designed to allow for a linear displacement of the surface sample up to 100 mm.
The horizontal translation of the sample is realized by a linear voice coil actuator (d), imposing the desired scanning velocity. In this manner, the sample is moved without any direct contact between the dynamic and static part of the actuator, i.e. without sliding vibrations. A high displacement resolution (0.5 µm) is obtained using a feed-back position control system and a nocontact linear encoder.
Figure 2. Experimental set up configuration used for the measurement on the real finger (a) and on the finger model (b), a scheme of the compliant system (c) and the finite element model for an applied force of 4 N and a displacement of 50 mm(d). In order to have a better control of the normal force applied by the finger and of the scanning speed, the fingertip is fixed while the scanned surface is moved by the sample translation. Two triaxial force transducers (e) are placed below the sample for measuring the global contact forces (used for the global friction coefficient calculus). The accelerometer, used in the preliminary tests, was mounted on the finger nail to measure the finger vibrations induced by the scanning with the test surface.
By definition, compliant systems transfer motion, force, or energy gaining their mobility from the deflection of flexible members rather than from movable joints. The system, allowing the linear displacement, is machined by Electric Discharge Machining (EDM) technique from a brute plate of martensitic stainless steel; in consequence, the total system dimensions (in this case 372x300 mm) are limited by the maximum working dimensions of the employed EDM machine. More specifically, the realization is developed in two steps: the shaping of the whole system obtained by EDM; and the refining of flexible joints to the whished dimensions using the same machining technique. The flexure hinge, a thin member that provides relative motion between two adjacent rigid members through flexing [12], is obtained machining a blank piece of material, thus avoiding the need for any sliding surface and any friction losses or noise from the joints; on the other hand it is very sensitive to fatigue and overloading. The most employed flexure hinges configurations are the circular, the corner filleted, the parabolic and the elliptical, each one characterized by specific properties that make it more suitable for a specific application. Considering the hinge geometry, the simplest ones are the circular and the corner filleted (fig 3). In cases like this, large deflection angles have to be reached and corner filleted flexure hinges, with respect to circular ones, permit to have a higher deflection for smaller system dimensions [13].
Figure. 3 Circular and corner filleted flexure hinge characteristic geometry. Employing flexible joints, the simplest way to obtain the motion of a horizontal rigid segment is to realize a table with four of them (green line in figure 2 c). With regard to the kinematics, the horizontal motion of this table (approximated by a parallelogram) is not perfectly linear; meaning that, the height of the final position of the table upper part is different from the starting one, and the described trajectory is parabolic. This height difference leads to force measurement errors and could cause a voice coil misalignment. To avoid this, it is possible to use a double table (double parallelogram) as shown in blue in fig.2 c. In fact, the double system permits to perform the desired horizontal displacement without any vertical motion because the two identical tables are placed in opposition canceling the final vertical displacement. To ensure the same motion angle for the two parallelograms (which constitute the compliant system), that guarantees the desired horizontal displacement avoiding vertical displacement of the upper segment, a lever (red in figure 2 c), connecting the two horizontal segments with the frame, is introduced in the central part of the system. For this configuration, the length (l) of the lever in which is inserted the hinge (determining the structure dimensions), the material (young modulus E and the fatigue limit stress for 107 cycles σadm), the maximum displacement (2f), and the hinge characteristics (h and lc) are related by the expression:
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Equation 2.1 represents the corner filleted hinge admissible displacement, and permits, once fixed the material, the desired displacement and the dimensions limits, to calculate the hinge dimensions h and lc. Several iterations of this process permit to optimize the system dimensions reducing costs and improving performances. For this specific application, the compliant system and its hinges have been dimensioned to have a linear displacement of the surface sample up to 100 mm (±50 mm, with respect to the equilibrium position). In order to obtain such excursion of the tangential displacement, taking into account the whole system dimensions limits, the flexible joints length is, depending on the joint position, 6 mm (parallelograms joints) or 20 mm (lever joints). Once designed the compliant system, a validation by FEM calculation has been performed by the commercial code ANSYS (Figure 2_d). The static analysis allows for the validation of the global stiffness of the system and the stresses distribution at the flexible joints. The final error calculated and measured on the non-planarity of the linear translation of the surface sample is about 20 µm, which is negligible for the performed tests. In addition to the measurements developed on the real finger, the TriboTouch permits to perform the same tests involving a fake finger (fig.2 b). In this way it is possible to analyze both the global vibrations (on the finger nail using the accelerometer) than the local ones (at the contact zone using a fast camera and the image cross-correlation technique), avoiding the need to analyze all the surface samples with the real finger. The fake finger is realized in RTV silicone which allows to reproduce fingerprints and fingertip shapes by molding technique. An aluminum lever, representing the finger bones, allows for fixing the finger on the set-up structure and confers stiffness to the finger. 3 TEST BENCH VALIDATION An accurate analysis of the vibrations induced from the scanning requires several tests involving a great variety of surface samples. Nevertheless, to validate the set-up and the measurement methodology, it has been decided to employ samples with a periodical roughness, with roughness groves perpendicular to the scanning direction; the vibration frequency can be found as a function of the scanning speed and roughness spatial period (width) [5, 14, 15]. In order to verify the negligible value of the parasitic noise coming from the set-up operation, the measurements of the acceleration on the fingernail have been performed during the sample translation with and without scanning between the fingertip and the surface sample. Specifically, in Fig. 4 a, the black line shows the vibration detected on the finger while it follows the sample without a relative motion between the two surfaces in contact and thus without any friction; the blue line shows the vibrations induced by friction when the finger stays fixed and the sample is moving. The parasitic noise from the set-up was found to be negligible with respect to the friction induced vibrations measured when scanning the surface, as confirmed, also from the two signals FFTs shown in figure 4 b. The spectrum due to the vibration of interest is considerably higher than the one of the parasitic noise. The measurements performed on samples with periodical roughness shows well defined frequency peaks that depend on the period of the roughness on the sample [5, 15]. As expected, the frequency peak values are a function of the relationship between the surface sample and
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Figure 4. An example of the comparison between the acquired acceleration signal of the vibrations induced by the fingertip/surface scanning and the parasite noise from the set-up functioning (a) and their FFTs (b). A preliminary test was performed with the sample fixed and with the scanning directly performed by the subject, which moves his finger to investigate the surface. In fact, in the every day life, when someone touches a surface no controlled speed movement system is employed. This preliminary test is employed as reference test to verify if the employed scanning speeds are compatible with the real one. Five different scanning velocities have been used, from 10 mm/s to 50 mm/s, representing the range of scanning speed normally employed by people judging a surface. Nine samples with sinusoidal roughness have been investigated, at different scanning speed, by different subjects, with the right hand index finger. The selected roughness range (0.64-5.2µm) covers surfaces perceived practically smooth up to surfaces with a well defined roughness. The periodical roughness is obtained by milling the surfaces with different milling cutters. The resulting spatial periods range from 0.15 to 2.17 mm. All the experiments took place at the same temperature 24°C and in the same environmental conditions. The skin was untreated, meaning that no moisturizers were employed, but, to be sure that the contact was always under the same conditions, the finger skin and the surface sample were cleaned up, only with alcohol, before each measurement. Only tests with slight variations of the normal force have been retained for the analysis. The applied normal force varies among the subjects [6, 7] in function of the gender and the age, and generally falls in the range 1±0.8 N. During the scanning of the sample the subject, standing close to the experimental set up, leans his elbow on a rigid planar surface to limit the unintentional movements during the measurements and to have a better force control. The accelerometer, mounted on the finger nail, detects the global vibrations but does not give information about the local vibrations (at finger/surface contact zone). So, to even obtain this addition information, accounting for the different points of measurement with respect to the fingertip surface, tests have been performed to obtain the finger transfer function by exciting the fingertip contact surface with a shaker and measuring the response with the accelerometer on the finger nail. The results show high reproducibility for the same subject and similar behaviour between different subjects [5, 14, 15]. So, dividing the obtained spectra by the finger transfer
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function it is possible to get the local vibrations. Regarding the global vibrations, figure 5-a shows the FFTs of the spectra of the acceleration measured on the finger for different scanning speeds on the same roughness sample, that has a roughness wavelength of 0.15 mm and a mean roughness of 1 µm. Increasing the scanning speed from 10 mm/s to 50 mm/s the peaks on the frequency spectra cover larger values and their amplitude rises. The same behaviour can be found for all the surface samples and for different subjects, confirming that the designed experimental set-up is suitable for measuring and investigating the induced vibrations. To validate the representatively of the measurements obtained by the fake finger, the same tests have been performed using both the real than the fake one. As shown in figure 5 b from the comparison between the vibrations FFTs coming out from the scanning of the real finger and the fake one, on the same sample for three different scanning speeds, the vibration spectra shows a good agreement of the results.
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Figure 5. Comparison between the acceleration FFTs obtained, on the same surface sample, scanning with the real finger used for the molding (a) and the fake finger (b).
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The designed apparatus allows for calculating the global friction coefficient between fingertip and surface, by monitoring the normal and tangential forces at the connection between surface sample and support. This will be useful, in further investigations, to characterize the link between the touch sensation and the energy dissipation. In figure 6 there is an example, for the surface sample employed in the measurements described previously, of the friction coefficient behaviour with respect to the scanning speed, while the normal force is maintained at 0.25 N.
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4 CONCLUSIONS This paper describes a unique test bench, the TriboTouch, which has been developed to reproduce the scanning action between finger and surface under controlled parameters, avoiding any other contact surface in sliding, i.e. preventing parasitic vibrations affecting the measurements. Using this new apparatus, the influences of scanning speed and roughness wavelength on the vibration spectra have been investigated [5]. For each contact surface, an increase in speed corresponds both to the shift of the acceleration spectrum toward higher frequencies and to larger magnitudes; for samples with periodic roughness, the frequency peak value increases linearly with an increase in the scanning velocity [fig 7 (a)]. Two different perception mechanisms are highlighted as a function of the roughness wavelength and vibration spectra: when the wavelength of the surface roughness is smaller or comparable to that of the fingerprint one, the surface roughness is perceived as a result of the vibrations induced by the finger scanning. When roughness wavelength is much larger than that of the fingerprint one, it is perceived as a quasistatic pressure distribution on the fingertip surface [fig 7 (b)]. These results agree with psychophysical and neurophysiologic analyses reported in literature and highlight the role of fingerprint roughness on the frequency filtering mechanism. 0.15 0.3 0.66 0.781 0.87 1.1 1.4 1.7 2.17
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A preliminary analysis of the spectra of vibrations induced by scanning textiles was also performed. The spectra of the friction induced vibrations obtained when scanning a textile sample show simultaneously: a vibration frequency peak, characteristic of the periodically rough surfaces, which is due to the texture of the fabric; an a wider frequency distribution, characteristic of isotropic roughness, which is due to the roughness of the textile fibres. The measurements reported in this paper and the results obtained by the TriboTouch set-up [5, 15] attest the effectiveness of the designed test bench.
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