Measurement of Skin Deformation Using Fingerprint Feature Tracking

Introduction

This project proposes a skin strain measurement technique that relies on the tracking of anatomical landmarks of a fingertip sliding over a transparent surface which can be flat or have simple geometrical features. The immediate motivation is the need to generate driving signals for the STReSS, a tactile display that relies on distributed lateral skin strain patterns to cause tactile sensations. The aim is to create tactile movies, i.e. driving signals, from observation of the fingerpad deformations during tactile exploration. This measurement technique may also help improve our understanding of the mechanical behavior of the fingerpad and of the relation between mechanical signals and tactile perception.

This web page provides a summary of the technique as well as a some interesting results. Readers are referred to [1] and [2] for more details.

Skin Strain Measurement Technique

Techniques adapted from the field of online fingerprinting are used to acquire high-contrast fingerprint images and extract salient features resulting from anatomical landmarks as they contact a surface (pores, valley endings, and valley bifurcations). Further processing involving the computation of a triangulation of these features is then used to evaluate skin strain variations over time. Please click on the following links to obtain more information about each aspect of the skin strain measurement technique.

Fingerprint Image Acquisition Feature Extraction Skin Strain Measurement

Experimental Results

Experiments were conducted with three types of surfaces: a surface with a bump, a surface with a hole, and a flat surface. All experiments were performed with the same fingertip. Each recording consists of 180 frames at 60 frames/second. Results are shown in the form of mpeg movies at a slowed-down rate of 15 frames/second.

Measurements obtained from movement over flat surfaces are generally difficult to interpret. The following two experiments provide interesting measurements obtained from image sequences for which a meaningful interpretation could be found. In the first example, a fingertip is pressed against the surface and rotated. In the second example, a fingertip is moved back and forth horizontally. Please click on the following links to see the results.

Rotation Lateral Movement

A second set of experiments was conducted with three contact surfaces: a flat surface, a surface with a bump, and a surface with a hole. The fingertip was moved from left to right at an average speed varying from 2.0 to 2.8 mm/s. While no pattern emerges from the flat surface, a tendency of compression can be observed on the left and a tendency of expansion on the right of the bump. The reverse can be observed in the case of a hole. Measuring the changes in edge length or area over a span of 10 frames provides cleaner results. Please click on the following links to see the results.

Flat Surface Surface with a bump Surface with a valley

Acknowledgment

Financial support from the Institute for Robotics and Intelligent Systems (IRIS) and from the Natural Sciences and Engineering Council of Canada (NSERC) is gratefully acknowledged.

References

  1. Vincent Levesque and Vincent Hayward, Experimental Evidence of Lateral Skin Strain During Tactile Exploration, Proc. Eurohaptics 2003, Dublin, Ireland, July 2003. click here to download
  2. Vincent Levesque, Measurement of Skin Deformation Using Fingerprint Feature Tracking, M.Eng. Thesis, McGill University, November 2002. click here to download
  3. Jerome Pasquero and Vincent Hayward, STReSS: A Practical Tactile Display System with One Millimeter Spatial Resolution and 700 Hz Refresh Rate, Proc. Eurohaptics 2003, Dublin, Ireland, July 2003. click here to download