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Characterization of the Mechanical Properties of the Human Fingerpad

A 2-DOF robot designed by Dr. Howe of the Harvard group was modified to serve as a "Tactile Stimulator" capable of delivering static and dynamic stimuli to the human fingerpad (Gulati, 1995; Gulati and Srinivasan, 1995). Three types of indentors (point, flat circular, and a flat plate) attached to the stimulator imposed a variety of constant velocity ramps (1 to 32 mm/s), depths of indentation (0.5 to 3mm), and sinusoids (0.25 mm to 0.5 mm amplitude; 0.125 to 16 Hz frequency) under displacement control (resolution ~ 20 microns). The resulting normal and shear forces were measured by a 2-axis force sensor (resolution ~ 10 mN). The results showed a pronounced nonlinear force-indentation depth relationship under both static and dynamic conditions, viscoelastic effects of force relaxation under constant depth of indentation, and hysteresis under sinusoidal displacements. There was wide variability in the magnitude of response for the five subjects who were tested, and their fingertip diameter or volume did not account for the observed variability. A piecewise linear, lumped parameter model with spring and dashpot elements was developed to identify the mechanical parameters causing the nonlinear response. The model predictions with only one set of parameters for each subject matched the empirical data well under a wide variety of stimuli. The model represents a compact description of the data and will be used to verify and tune our finite element models of the fingertip.

Force response of the human fingerpad to shear displacement

The 2-DOF Tactile Stimulator was also used to deliver shear displacement ramp (0.5 to 16 mm/sec for a total shear displacement of 7mm) at various depths of indentation of the fingerpad (0.5 to 3 mm) by a flat, smooth Aluminum plate (Towles and Srinivasan, 1994). Only one subject has been tested so far under these stimuli, and the resulting data has been analyzed with a view towards fine tuning the experimental protocol and parameters. The results show that at each depth of indentation, the shear displacement initially caused increasing skin stretch and shear force, followed by slipping of the plate across the skin surface. The shear force-shear displacement was almost linear and slip occurred at around 3mm shear displacement at all velocities. Low velocities tended to cause stick-slip (as indicated by oscillatory shear force during slip), whereas the shear force decreased smoothly at higher velocities. At increasing depths of indentation, slip occurred at larger shear displacements, as is to be expected. The coefficient of static friction was obtained by measuring the slope of the normal and shear forces at the incipience of slip for a given shear velocity. To a first approximation it was found to be independent of shear velocity. More experiments on different subjects are being initiated.

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Last Updated: May 8, 2002 1:45 PM Comments: David Schloerb