From nature to MEMS: towards the detection-limit of crickets' hair sensors

Abstract

ABSTRACT Crickets use highly sensitive mechanoreceptor hairs to detect approaching spiders. The high sensitivity of these hairs enables perceiving tiny air-movements which are only just distinguishable from noise. This forms our source of inspiration to design sensitive arrays made of artificial hair sensors for flow pattern observation i.e. Flow camera. The realization of such high-sensitive hair se nsor requires designs with low thermo-mechanical noise to match the detection-limit of crickets’ hairs. Here we investigate the damping factor in our artificial hair-sensor using different models as it is the source of the thermo-mechanical noise in MEMS structures. The results show th at the damping factor estimated in air is in the range of 10 -12 N.m/rad.s -1 which translates into a 52 m/s threshold flow velocity. Keywords: Bio-mimetic, artificial hair-sensor, detection-limit, squeeze film damping 1. INTRODUCTION In the last decade, the mechano-sensory hair system of cr ickets has been a common research subject between biologists and engineers. This group of hairs, as found on arthropods, notably on spiders and crickets, are among the most energy-efficient flow sensory systems appearing in nature. A large canopy of mechano-sensory hairs residing on the cerci of crickets forms the sensing part of the cricket’s escape mechanism e.g. from spider-attacks [1]. Air movement due to approaching predator causes the cricket to rurn rapidly away from the stimulus [2,3]. The large numbers of hairs and associated hair density, the mechanical properties of the hairs, their directivity and the accompanying neural system combine to form an e ffective system capable of extracting the aerodyna mic representations of animal movements with high spatial resolution. This enables the cricket to perceive flow signals at thermal noise levels (as low as 30 m/s [4]) and, using canopies of hairs, to discern flow phenomena at high spatial resolution without interfering with the flow medium [5]. Inspired by crickets and with the assistance of MEMS technological advances, single and arrays of artificial hair flow sensors have been designed and implemented successfully by different research groups [6-8]. Different structures with various transduction mechanisms have been used in the literature for designing flow sensors, for instance, capacitive, piezoresistive, thermal or optical sensing techniques. Dijkstra et al. fabricated flow-sensor arrays imitating the filiform hairs of crickets [6]. Surface micro-machining technology has been used to fabricate suspended silicon nitride membranes and hairs were made by a repeated lithography process to form double layers of SU-8 (negative photoresist). This approach of biomimetic hair flow sensing forms the core of our study. Figure 1 shows the structure of the artificial hair flow sensor used in this st udy and its source of inspiration. In this contribution, we investigate the thermo-mechanical noise (caused by damping) in our artificial hair-flow sensor. This reveals the limits of the design of our hair sensor to wards matching the detection-limit of crickets’ hair sensors. The aim is to develop highly-sensitive array system made of artificial hair flow-sensors to be applied in flow pattern measurements. The realization of such sensors with high se nsitivities requires designs with both low thermo-mechanical noise and high-resolution of angular displacement.