Marcel Dijkstra

Mems based hair flow-sensors as model systems for acoustic perception studies

Arrays of MEMS fabricated flow sensors inspired by the acoustic flow-sensitive hairs found on the cerci of crickets have been designed, fabricated and characterized. The hairs consist of up to 1xa0mm long SU-8 structures mounted on suspended membranes with normal translational and rotational degrees of freedom. Electrodes on the membrane and on the substrate form variable capacitors, allowing for capacitive read-out. Capacitance versus voltage, frequency dependence and directional sensitivity measurements have been successfully carried out on fabricated sensor arrays, showing the viability of the concept. The sensors form a model system allowing for investigations on sensory acoustics by their arrayed nature, their adaptivity via electrostatic interaction (frequency tuning and parametric amplification) and their susceptibility to noise (stochastic resonance).

Artificial sensory hairs based on the flow sensitive receptor hairs of crickets

This paper presents the modelling, design, fabrication and characterization of flow sensors based on the wind-receptor hairs of crickets. Cricket sensory hairs are highly sensitive to drag-forces exerted on the hair shaft. Artificial sensory hairs have been realized in SU-8 on suspended SixNy membranes. The movement of the membranes is detected capacitively. Capacitance versus voltage, frequency dependence and directional sensitivity measurements have been successfully carried out on fabricated sensor arrays, showing the viability of the concept.

Biomimetic micromechanical adaptive flow-sensor arrays

We report current developments in biomimetic flow-sensors based on flow sensitive mechano-sensors of crickets. Crickets have one form of acoustic sensing evolved in the form of mechanoreceptive sensory hairs. These filiform hairs are highly perceptive to low-frequency sound with energy sensitivities close to thermal threshold. In this work we describe hair-sensors fabricated by a combination of sacrificial poly-silicon technology, to form silicon-nitride suspended membranes, and SU8 polymer processing for fabrication of hairs with diameters of about 50 &mgr;m and up to 1 mm length. The membranes have thin chromium electrodes on top forming variable capacitors with the substrate that allow for capacitive read-out. Previously these sensors have been shown to exhibit acoustic sensitivity. Like for the crickets, the MEMS hair-sensors are positioned on elongated structures, resembling the cercus of crickets. In this work we present optical measurements on acoustically and electrostatically excited hair-sensors. We present adaptive control of flow-sensitivity and resonance frequency by electrostatic spring stiffness softening. Experimental data and simple analytical models derived from transduction theory are shown to exhibit good correspondence, both confirming theory and the applicability of the presented approach towards adaptation.

Analysis of the performance of a particle velocity sensor between two cylindrical obstructions

The performance of an acoustic particle velocity sensor that is placed between two cylindrical objects has been analyzed both analytically and by means of finite volume simulations on fluid dynamics. The results are compared with acoustic experiments that show a large magnification of the output signal of the particle velocity sensor due to the mounting of the sensor between two cylinders. The influences of this construction consist of an attenuation of particle velocities at frequencies below a few hertz, whereas signals in the higher frequency range are amplified, up to approximately three times (10 dB) in a frequency range between 50 and 1000 Hz. The theoretical analysis is based on the derivation of the stream function for the situation of two long cylinders immersed in an oscillating incompressible viscous fluid, at low Reynolds numbers. The results lead to an improved insight into the effects of viscosity and fluid flow that play a role in acoustic measurements and open the way for further optimization of the sensitivity of the sensor.

Drag Force Actuated Bistable Microswitches for Flow Sensing

This paper presents bistable microswitches with Au contacts with the aim to combine them with artificial hairs for flow sensing. The Au contacts are applied on both ends of a silicon nitride beam, suspended by a torsional bar at its center. The beam is provided with electrodes for electrostatic actuation, which were used for characterization and can also be used for adaptive control of the mechanical properties of the flow sensor. The electrodes have been actuated in anti-phase to drive the microswitch similarly to an astable multivibrator. Single-sided switching has been measured up to 10 kHz actuation frequency.

Miniaturised Prandtl tube with integrated pressure sensors for micro-thruster plume characterisation

A miniaturised Prandtl-tube sensor incorporating a 6 mm long 40 μm diameter microchannel with integrated pressure sensors has been realised. The sensor has been designed for the characterisation of rarefied plume flow from a MEMS-based monopropellant propulsion system for high-accuracy attitude control of satellites. The 4.5 × 2.5 mm sensor chip incorporates 400 mbar full-scale capacitive pressure sensors. The capacitive pressure transducer is created by merging several microchannels into a rectangular pressure membrane, with outward-facing comb-fingers hinging on the microchannel sidewall. Additionally, thermistors can measure the temperature profile on the Prandtl tube and an integrated Pirani sensor can optionally measure vacuum pressures below 10 mbar. An electronics board and stainless steel probe, from which only the Prandtl tube protrudes has been realised. Initial plume measurements of dynamic pressure on the Prandtl tube have been obtained by flowing air from a metal tube with 0.7 mm diameter, demonstrating the feasibility of the miniaturised Prandtl-tube sensor.