Development of a bio-inspired MEMS based tactile sensor array for an artificial finger

Abstract

In this thesis, the design, fabrication and characterisation of a bio-inspired microelectromechanical systems (MEMS) based tactile sensor array is presented. A vast amount of research has been carried out in the area of tactile sensing and various transduction methods have been explored. However, currently no device exists with a performance comparable to that of the biological tactile sensors of the human fingertip in terms of robustness, sensitivity, spatial resolution and dynamic performance. The sensors developed in this work employ the principles of electrical capacitance and are fabricated from commercially available siliconon- oxide wafers using simple process steps. Each sensor is formed from two plates of highly conductive silicon separated by an air-gap formed from sacrificial etching of the oxide layer. Deflection of the 2 \(\mu\)m thick upper plate of the sensor as a result of applied mechanical stimulus, causes a change in capacitance which is the output of the sensor. Within the array, the individual sensors are spaced 150 \(\mu\)m apart (centre-centre pitch of 570 \(\mu\)m) and therefore offer the potential for high spatial resolution. To protect the sensor array from mechanical shock and provide skin like compliance, the use of suitable packaging materials was explored. The use of poly dimethyl siloxane (PDMS) as a suitable skin-like material was demonstrated. Modification of the surface topography of the packaging layer to include ’fingerprint’ like features was explored and its benefits highlighted. Sensor characterisation experiments revealed that the sensing device was sufficiently sensitive to allow the discrimination of different textures (with feature spacing down to 0.2 mm) through tests conducted using gratings varying in spatial periodicity and fabrics. Based on the results, the sensors can be used as an analogue of the slowly adapting tactile receptors (Merkel disks) for robotic finger applications.