A 3D-Printed Soft Fingertip Sensor for Providing Information about Normal and Shear Components of Interaction Forces

Gerjan Wolterink,Bert-Jan van Beijnum,Remco Sanders,Gijs Krijnen,Peter Veltink

doi:10.3390/s21134271

Abstract

Sensing of the interaction forces at fingertips is of great value in assessment and rehabilitation therapy. Current force sensors are not compliant to the fingertip tissue and result in loss of touch sensation of the user. This work shows the development and characterization of a flexible fully-3D-printed piezoresistive shear and normal force sensor that uses the mechanical deformation of the finger tissue. Two prototypes of the sensing structure are evaluated using a finite element model and a measurement setup that applies normal and shear forces up to 10 N on a fingertip phantom placed inside the sensing structure, which is fixed to prevent slippage. Furthermore, the relation between strain (rate) and resistance of the conductive TPU, used for the strain gauges, is characterized. The applied normal and shear force components of the 3D-printed sensing structure can be partly separated. FEM analysis showed that the output of the sensor is largely related to the sensor geometry and location of the strain gauges. Furthermore, the conductive TPU that was used has a negative gauge factor for the strain range used in this study and might cause non-linear behaviors in the sensor output.

Highlights

Measurement of the interaction forces between the fingertips and the external environment is of great value in upper extremity assessment and for interaction in rehabilitation therapy where patients need to relearn motor tasks [1]
We note that the initial resistance drop at low strain values may be related to alignment of conductive carbon black particles in the TPU material, whereas at higher strains breaking down of these networks may be causing the increase in resistance, as proposed in [8]
It is believed that the mobility of polymer chains causes the formation and breakdown of conductive networks [20], where at higher strain rates the resistance will increase more by this effect compared to lower strain rates since the breakdown effect is more dominant at high strain rates [20,21]

Summary

Introduction

Measurement of the interaction forces between the fingertips and the external environment is of great value in upper extremity assessment and for interaction in rehabilitation therapy where patients need to relearn motor tasks [1] While handling objects, these interaction forces can reach up to 50 N for normal and up to 10 N for shear forces, where the high dynamic range for resolving both forces is below 10 N [2,3]. Most sensors are mainly made of stiff materials leading to the loss of touch sensation of the user These sensors are not ideally adapted to the shape and stiffness of the fingertip, leading to poor sensor-to-skin attachment. Due to this poor contact the sensors tend to shift by external forces [4]. In practice this issue has presented itself in the form of limited usefulness and reliability of current force sensors [5]

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