Analysis of the biodynamic interaction between the fingertip and probe in the vibrotactile tests: the influences of the probe/fingertip contact orientation and static indentation.

Vibrotactile thresholds at the fingertips are affected by a number of individual, environmental, and testing factors. In the current study, we theoretically analyzed the effects of the contact orientation of the probe on the fingertip and the static pre-indentation on the dynamic deformation of the soft tissues of the fingertip in the vibrotactile tests using a nonlinear finite element model. The fingertip considered in the 3D finite element model is the distal phalanx, the portion from the distal end to the distal interphalangeal (DIP) joint articulation. The fingertip is contacted by the probe at four different contact locations, which are regulated by contact angles (15 degrees, 30 degrees, 45 degrees, and 60 degrees), and three different pre-indentations (0.5, 1.0, and 1.5 mm). The model predictions indicated that the average spatial summation of the vibration displacement (SVD) at the fingertip depends on the static pre-indentation and the probe/indentor contact orientation; although the resonance characteristics of the fingertip are not affected by either the pre-indentation or the contact location. The location-dependence of the vibration exposure factors at the fingertip was found to increase with increasing static pre-indentation. At a static indentation of 1.5 mm, the test condition specified in the ISO-13091-1 standard, the values of the SVDs determined at different probe/fingertip contact orientations differ as much as 125%. Since the dynamic displacements of the soft tissues are believed to affect the vibrotactile threshold, the current results suggest that the contact orientation of the probe on the fingertip should be strictly defined and restricted to obtain reliable results in the vibrotactile perception threshold tests.